Hi all, I have a set of Klipsch La Scala AL5 speakers (nominally rated at 105 dB efficiency, IRL probably lower), and a couple of different Decware triode amp based setups. I feel like I've discovered that Decware's amps might not be a great fit for speakers this efficient, and I wanted to get some feedback on the community from this before I jump to conclusions and totally re-think my setup. Also, if my findings are generally correct, this post could also act as a PSA for people to think hard before chasing Decware amps to pair with very high efficiency speakers.

I'll first say that when I first got my La Scalas which were my dream speaker at the time, I did research and found that many people were saying that it's a no-brainer to pair high-efficiency speakers like this with good low power SET amps -- a perfect match. This is around the time that Decware's wait list really started to blow up (thanks partly to Andrew Robinson, who highly sung the praise of the SE84UFO with his La Scala AL5s). I decided to jump in and see what the fuss was about.

Since then I've been able to get my hands on two different Decware amps to pair with my La Scalas, with limited success.

1) A pair of SE84UFO25s, which I have run in three different configurations: a) just normal stereo from a single amp, b) unbalanced dual mono, and c) fully differential balanced mono (in this case fed balanced signal from a ZTPRE)

2) SEWE300B (just arrived last week!) running in standard stereo operation without any pre-amp.

I'll start by saying that everything that everyone has said about how nice these amps sound is true, they're lovely. But in nearly all cases, I'm getting what I think is an untenably loud 60 Hz hum from my speakers and subs -- this hum is about 46 dB at my listening position, about 10 feet from my speakers. While this isn't noticeable when playing many different genres of music at sufficient SPL, it is ever present in quiet moments, and for example when listening to classical music (either solo piano or larger orchestral works) instruments fade into the 60 Hz hum instead of into silence which I think really hurts the music.

Of course, I've done research on these forums and there are many threads about hum and how to, in some cases, eliminate it. I've also had the pleasure of having a few long phone conversations with Steve about it, what my expectations should be, and how to troubleshoot it.

Often, hum is attributed to ground loops in your signal chain, and many people are able to substantially reduce and solve hum in their setup by figuring out which components are causing the ground loops, using DC blockers, etc etc. Based on my testing, I do not believe that I have any ground loop issues, or at least, by far the main contribution to the hum is not from ground loops. This belief is based on the fact that with any of the three amps (either of the UFO25s or the 300B), if I unplug absolutely everything from all outlets in the room, only plug the amp into an outlet, and then connect a speaker, the hum is there. It is completely unaffected by whether or not any input is connected to the amp, completely unaffected by whether or not I plug other things into other outlets, and is also completely unaffected by the volume knob on the amp -- it hums just as loud at zero gain as it does at full gain.

In my conversations with Steve, I explained my situation, and he explained that there is definitely inherent hum in these amps that is part of the nature of the design. He very carefully voices his amps and has specific design targets in mind when he does so, and although he could completely eliminate the hum, the amp wouldn't sound the way he wants it to if he did so. He said (and I may be misquoting here, sorry if I get this wrong, Steve) that he designs it such that when using ~100dB efficient speakers, whatever inherent hum exists is negligible/unnoticeable at a normal listening position, but detectable if you go right up the driver. Unfortunately, with my La Scalas, that is far from the case. The conclusion that we seemed to get to in our conversations was that the hum I'm experiencing is inescapable if I'm going to pair speakers this efficient with these amps.

This is a bummer. There's so much I like about the sound of my UFO25s and of the SEWE300B, but it honestly makes it hard to listen to certain genres of music (e.g. classical music with high dynamic range) without being bothered, and it also makes it that having the amp on in the background playing music at a social gathering is annoying, because the hum is constant and totally noticeable by everyone in the room.

There is only one situation in which there is no hum: running the pair UFO25s in differential balanced mono. This isn't a full solution, though, as it prevents me from hooking up my REL 212/sx subs using high level connections, which is frustrating (have confirmed this with Steve). But it really gives me a taste for how good these amps can be, and it really motivates me to see if there is any way to enjoy my SEWE300B without the hum ruining it for me.

What I would like to ask is whether anyone else has had any similar experiences, and have figured out a way to fix it? I guess I'm specifically asking people who have paired Decware UFO or 300B amps with La Scalas or similarly efficient speakers without having audible 60 Hz hum at listening position? I'm holding out some sliver of hope that perhaps I misinterpreted or misunderstood aspects of my discussions with Steve when I was troubleshooting, and there is a way forward! And if not, I really would like to have my experience serve as a warning to people who are on the waiting list for one of these amps to pair it with a pair of La Scalas or Klipschorns, because if this is really an unfixable issue, these people might end up disappointed with their purchase.

One thing that gives me hope that there's a way forward here is that Andrew Robinson gushed about the pairing of his La Scalas with his SE84UFO, and I have a really hard time believing that he would have done so if he was getting the same loudness of hum that I am, or at least would have mentioned it as a caveat if he did.

I hope that I've faithfully represented my experience and the content of my discussions with Steve, but am happy to clarify further, or hear clarifications from Steve or anyone else if not! It's always been a delight chatting with Steve about this, and I think he makes great amps, and I hope my post does not come off as an accusation of false advertising in any sense on his part.

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46 dB hum at the listening position would indeed be a problem, I feel your pain. I have a pair of UFO25s, configured as balanced monos, and they are dead quiet. Like heat death of the universe quiet. I have 95 dB speakers, but I feel like I could certainly hear 56 dB hum if it were present. The same holds true for the UFOs I had previously.


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This is interesting. If feel like if you aren't getting hum with balanced mono, then the hum isn't inherent in the amps themselves. Did Steve have an opinion on that? To me it sounds like you are picking up noise, maybe RFI, that is canceled out with balanced.

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What if you connect the amp like it is balanced? I.e. connect the left and right positives to the speakers and short the negatives.

How long are your speaker cables? Sometimes they can act like an antenna can collect RFI. And that is a problem that balanced fixes.

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Long! 15 ft and 35 ft due to where equipment is vs speakers. I know this isn't ideal, and shorter runs are better, but it was never obvious to me that it could cause this big an issue. I've got some speaker cable I can experiment with by making a short run and comparing. The high level cables feeding my subs are I think ~30 ft and are the standard ones that REL supplies in the box.

Interesting food for thought in Andrew Robinson's La Scala AL5 review: he goes out of his way to make the point that due to the La Scala's efficiency, some amps aren't a great pairing because they're inherently noisier and the high efficiency of the La Scala's amplifies that in a way that might be unbearable. He suggested for example that pairing with the Naim Uniti Atom (which I also have) produces undesirable hiss and isn't a great match. On the other hand he says that the SE84UFO2 is a perfect match. The hum I get from my UFO25s on my La Scala's is way worse than any background noise/hiss I hear when I connect the Naim to my La Scala's, suggesting that somehow he didn't get any noticeable hum from his UFO2 (otherwise he very likely would have mentioned it).

We've had our Klipsch chorus II's since 1998. They are listed as 101 db, 1 watt/1 meter. We have had very low or even zero hum hooked up to our Zen UFO amp. Have to have our heads right next to the drivers of the speakers. And that's with no music source playing , or even when extremely low in sound level. 3 feet away, no apparent hum at all/ all sound levels.. So not sure it's a sensitivity problem. At least not in our case~ Have great evening~ Mike    edit: I remembered we used to have some hum due to our sub. a 3 prong plug adapter to a two prong cleaned that right up. In fact..it's still hooked up that way after having it since 2007. It's an HSU vtf-3. The original front firing version. It was mentioned in the manual to use the supplied adapter if needed.  Your amount of hum would drive me nuts. I hope it gets figured out. Such amazing sounding amps

You may have discussed or tried this, but I remember having a hum on my Zen amp 25th and receiving advice about using a grounding wire from a speaker terminal to the transformer and the hum disappeared.  I’ll confirm the wiring when I get back home, but maybe someone else can confirm this “solution” in the meantime.  Best of luck!

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I feel like this is the clue, but I'm not smart enough to figure it out. Since the hum is there without any input, or with very short speaker cables it is clearly coming from the amp. And since differencing the L and R channels removes it, it is the same in both channels. Very frustrating.

I'll also add that in our previous discussions of hum, it really did sound like Steve thought that this was just a speaker efficiency problem and was the nature of the beast, and it didn't sound like he thought that hunting down electrical issues or trying power conditioners etc would help. But I guess I'm not totally sure that I conveyed to him just how loud the hum is. Would be curious to hear what he thinks of this discussion, and I might try to schedule another call to discuss my SEWE300B now that it's here and humming louder than ever.

Just curious , It looks like you also have a Rachael , have you swapped that amp in directly and observed hum level ??

One final observation: the hum from the SEWE300B, when the hum knobs on the amp are optimized, actually appears to be 120 Hz, not 60 Hz. When the hum knobs on the amp are moved from their optimized positions, interestingly, 60 Hz hum seems to get imposed on top of the baseline 120 Hz hum. Anyway, problem in principle seems to be the same?

The only useful info I can add is I use Tekton Pendragons 96db and a UFO. With the UFO at 9 out of 10 on the volume I get an audible hum in my seat. I generally listen between 1/2 and 3/4 so it doesn’t bother me. Ear next to speaker, I can hear hum starting about 2/3 volume. I don’t hear that in my seat or it would drive me absolutely bananas. I hope you find a solution soon.

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Thanks for chiming in! I'm wondering if you happen to know what the sensitivity / efficiency spec for vintage La Scalas is? I wonder if it has changed in the modern ones or has stayed the same over the years.



Also, the loudness of my hum is totally independent of the gain setting, so, it actually makes low level listening worse

Thanks Mike, looks like it's about the same at 104 dB.

Will be chatting with Steve either today or Thursday, let's see what he thinks!

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I think one option that was suggested earlier was to get a balanced isolation transformer. I'm kinda surprised Steve didn't recommend that, he as before on the message board. There are a bunch of options, Decware makes one:

https://www.decwareproducts.com/zlc

I got mine from ebay, aka China or you can try these guys:

https://toroid.com/

FWIW I tried buying from them before ebay, but the wait was too long.

The transformer would isolate you from noise and/or DC on the mains power, but I'm assuming there is a class of problems that would require professional intervention. Full disclosure, my knowledge of home electrical is largely theoretical so JMO, YMMV and all that.

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Yeah I think I'm going to try this, gonna look for a used one that I can easily re-sell without a loss if it doesn't help.

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Yes, true. But as mentioned before, all of these amps are designed differently and will have different levels of inherent hum. My understanding is that 300B > UFO25 > Rachael in terms of internal hum, and I guess the theory is that whatever problem I have going on raises the hum level of all three, but the Rachael having less internal hum than the other two means it's still not bad even with whatever issue I have. I also have had other tube amps in this home that don't hum at all. In any case, your suggestion could still be worthwhile

Well, the good news is that a local electrician was able to come by today, but the bad news is that they said everything looks good and there are no issues with the house's wiring or grounding. He looked at the breaker panel, found where the actual ground rod is, and said everything looked fine. Tested one outlet, said everything looked good.

This leaves me in a tough spot. Steve seems sure the problem is not the amp, electrician seems sure the problem's not the wiring.. what else is there?

One desperate idea I have: I can bring the 300B to a local hifi space where they've got La Scalas set up, and see if we get the dreaded hum there.

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Thanks for describing your situation! Sounds like you had a ground loop. Unfortunately, the hum is the same even if literally the only thing plugged in to any outlet in the room is the amplifier. Also tried plugging everything into the same powerstrip etc. I do really wish it were a ground loop!

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I have not, I only have a single new pair of WE 300Bs that I bought with the amp. But based on the fact that my two different UFO25s hum almost as much as the 300B amp, though, even if a different pair of 300Bs did reduce the hum a bit, it would be unlikely to reduce to listenable level :/ also, the hum is identical in either channel, so both would have to be defective in the same way for that to be the issue?

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I'll definitely try a balanced transformer if I can get my hands on one I did call Upscale to ask about electricians but they just said they don't know anybody lol  [smiley=icqlite19.png] also tried many power cords and many outlets, with no change.

but with everything that's ruled out, hopefully we get closer!

Thanks, Kamran!! KingRex looks really promising I'll reach out to him tomorrow. The electrician that came by today was pretty quick about it, maybe a bit dismissive, and was a bit hesitant to bother testing my outlets after looking at the panel and ground pole -- seemed a bit incredulous that I called him out there to begin with. Sounds like KingRex will at least be patient in thinking about it.

As for the tubes, that's pretty interesting! And it's annoying to hear that WE didn't take both tubes back. It couldn't hurt to try a cheap pair of chinese 300Bs. I actually just tried configuring the SEWE300B in differential mono and the hum totally dropped to a very very faint background noise, so that might be what you're talking about Difference between single speaker hooked up to amp in normal stereo vs differential mono was 10 dB! Not a surprising data point though, given my experience with the UFO25.

At this point, if it were possible for me to get a second SEWE300B to run differential.. I'd consider it lol.

Considering everything you've done so far, (and I might have missed this in this long thread), I guess the first thing I'd do is to run a completely separate line from your breaker panel to a new outlet.  Sorry that you don't have a very interested local electrician, but perhaps you need to find someone else who can physically come to your place and put a bit more effort into helping you.  Dealing with old houses and their systems can be a nightmare sometimes.  Getting the isolation transformer should certainly point you in the right direction to hopefully getting some resolution.

I have a "PT-500 Toroidal Balanced Isolation Transformer" that is all black, but otherwise, the case shape and materials, and the inside look like the following. I put some better caps in it across the AC, which helped, but it seems nicely made, and sounded pretty good. https://www.ebay.com/itm/115706292755?itmmeta=01HQ6BA875BK56JSXYTNWJF0K9&hash=it...

Also this is a pretty interesting short thread, the guy who started the thread having some interesting points.

https://www.diyaudio.com/community/threads/antek-balanced-isolation-transformer-...

If you want a high quality American made solution from the maker of Goertz audio cables

https://www.bridgeportmagnetics.com/bmg-product/second-generation-balanced-power...

Good luck.       Crazy Bill

P.S. Check out my thread       ZP3  A Humdinger !

Had huge hum problems with my new ZP3 that simply didn't exist with the phonostage it replaced, a Sonic Frontiers Phono 1 w/ SE+ upgrades. Put in the ZP3 with everything else exactly the same, and unlistenable hum. It was so bad I decided to investigate other RIAA preamps, so I ordered an iFi Audio RIAA2 unit with upgraded power supplies, and a plug in noise filter that also checked the status of the AC power.

To make a long story short, checked the receptacle and it showed a bad ground. Mind you, using standard electrician's testers ( like the guy who came to your house ) showed everything was perfect. But the iFi tester showed bad ground.

Had another outlet on the other side of the living room. On the same circuit as the bad one but it tested good. So I ran a 12/3 extension cord from the good outlet and powered up my system.

Nirvana. Everything sounded great and was quiet.

Again, good luck.

Sorry for the lack of response, been pulled into work! I'll be able to update tomorrow

I have two units coming in for repair and each of them is a hum problem. Don't feel alone. One unit (Carver C-9) looks to have a jack that slanted on the rear panel, so I am betting that is a very possible issue in that case. The other is actually a preamp that I built! Maybe shipping problem? Dunno. It definitely seems to be more of a challenge. There are times that 'how' the components are connected to the AC power matters, i.e. a ground loop. One of the suggestions to even use a different outlet (on a different circuit of AC) will give you an idea. Unfortunately, it is true that many service people out there won't really investigate past what a meter tells them. Or worse yet, what a little plug identifier that is used to determine polarity, ground and earth would tell them. Glad that thought of tightening the contacts at the panel. A lot of grief can come from there. Same with every outlet in the house. That is one of the first things that I do if I move into a new place most AC outlets are crap and have been installed poorly in some cases. Each outlet in a string of outlets can cause a problem. As you go along make a check off list and order that things were done. It will help you in the end.

I've had 120hz hum issues with my SE84UFO25 as well (see this thread: https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1621262294/50#58 ) and as far as I can tell, it's power supply ripple, an inherent byproduct of converting AC to DC. I've got another tube amp that also has some ripple but it's lessened by having a choke on the power supply to the point where it's not audible from my listening position (whereas the UFO, I can hear it). If it is ripple, all the power regenerators in the world won't do anything, because at the end of the day its the smoothing out of the AC waveform itself that's the issue... as long as you have AC going in (which, you do), you've got the potential for ripple. There are ways to mitigate it within the design of the amp (choke, capacitors), but those mitigations can't easily be changed/swapped (other parts of the amp rely on the specs of these things to operate correctly). It's definitely frustrating, I do wonder why some people seem to hear it and others report their amps being dead silent... My suspicion is the all hum, but if your room is big enough and you're far enough away, you won't hear it since it is fairly quiet. If some truly are dead silent, well, then, I'd love to know how to get mine to be that way too!

Mike - If you put your ear right next to the speaker, is it still dead silent? E.g. if i use my phone's decibel meter, I get about 55 dB right next to the driver (vs ~30 dB for just the room). Where I sit it's barely audible, but it is there. If it's legit silent at the driver (I have a solid state amp that is), then I'm flummoxed and would love to know why some are silent and others aren't. I've tried all manner of power filtering/cords/different circuits in entirely different houses/buildings, so I don't think it's power related, but who knows!

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Yeah, I have also wondered about this because I've had both UFOs and UFO25s and they were/are silent. Heat death of the universe silent. Steve says the 120 Hz hum spec for a UFO25 is -58 dB, relative to some signal level, lets say 1 watt. So if you have 100 dB speaks that makes the hum 42 dB @ 1 meter. If your speakers are 90 dB that makes it 32. You don't say (or I missed) what kind of speakers you have. I also wonder if you have calibrated your iphone dB meter? My understanding is the meter is pretty good, but it needs to calibrated if you want to trust the actual numbers. I don't have any answers, esp. if you have thoroughly investigated your power. What about tubes, esp. the rectifier? They can matter too. It also may be some builds are just quieter than others, although I have no technical justification for that.

It may be the power transform bell covers screws that pass through the laminations creating hum. It happened on my zkit Edcor output transformers. Had hum until the covers and screws are removed.

Just saying.

If memory serves correctly it was 60hz. My thought at the time was those screws passing through laminations created an dipole antenna of sorts.

Hi Sam. I waited for Kathy to get back because I wanted her to listen for hum as well with me. Our two channel system consists of our new Zen ufo amp, hooked to Klipsch ChorusII speakers (101 db, 1 watt-1 meter). The only source is our Oppo bdp-103 player. We put an ear right next to (almost touching) all three drivers, ( tweet, mid and 15 inch woofer) and heard absolutely zero hum. This is the first time we listened for it that closely before. I am currently playing an ELO Flac file from a usb drive on the front slot of the oppo. No hum on cd's played either. I really hope that there is a way to mitigate any of that for you. I will check on another, different day too. We do not live in or around a city, I don't know if that makes any difference though with power lines.. We listened with the volume level on the amp up and down. Music was paused of course. I wish I had another idea other than the ones folks have already suggested. All the very best Sam ~Mike

Hi Mike, Thanks for thoroughly investigating your amplifier's hum, or, better said in your case, the absolute absence of the amplifier hum. Bringing in another pair of ears to listen added to the validity of your results for me. And congratulations on how hum-less your system performs. I thought that all tube amps probably hummed to some extent, but your results argue against that conclusion.

Sorry to hear about this frustrating problem!

I think it would be interesting to see what happens when you use your Sarah300b in a different system.  Bring it to a Klipsch dealer or a friends house.
At least then you can answer the question if there is something intrinsic to the amplifiers causing the problem.  Tubes, transformers etc. My bet is that there isn't something inherent in Decware designs compared to other tube amps.

At the very least, if you decide to sell the Decware's you can say they are totally silent in other peoples setup.

-Karl

Thought about that too Karl. Folks in large cities..or near one, would sure be worth a try. If he lived near, I'd ask him to bring the amp over as our setup has absolutely zero hum. and that's with a sub hooked up too.  ChorusII's at 101db I'd think would work. But....we are in the sticks people... Like you said, maybe a Klipsch dealer, or friends home. We had a sub that really screwed with us for quite some time several years ago. Hummed like a sob. It all started after a thunderstorm one night. Lightning all over. I changed out a long sub cable  (35 footer for a blue jeans cable one) a huge difference. But still some hum left. Replaced the plate amp and poof. Almost completely silent now. Have to put my stupid head into the driver to hear much at all...I hope mk gets this going as it should sound nuts (good!). I would have loved a set of LaSacalla's at one time..but were always just a bit too big to fit our front wall area. That strike shook me up that night..wasn't ready for it that close.

MK, I may have missed it.  Where are you located?

Too bad.  I have a couple pair of speakers you could have tried.

I can’t wait to hear your results of this test! Please, keep us posted!

On that note, if there's anyone in the Boston area with an SE84UFO25 that's dead silent, I'd love to meet up and compare amps, see if it's dead silent in my setup and/or mine hums in yours!

Also, add in with the above that everyone hears a little differently and tends to be inexact with language in different ways, and I'd expect that to account for small discrepancies in experiences.

Same thing Sam said goes for me in socal: if anyone has a UFO25 that they believe is truly dead silent (which by what I've suggested above wouldn't be possible), would love to compare with my two UFO25 amps!

Agreed! If there really is variation to the degree that you're hearing hum on low 90s efficiency but CAJames is really getting silence, and it's a difference in the amps themselves, then this does need Steve's attention. Having an amp like CAJames' could mean the difference between being able to comfortably use a single UFO25 on my La Scalas vs needing having to have a pair in balanced mono for it to be useable. But based on my interactions with Steve, I would be surprised if he agreed that any UFO25 could be "heat death of the universe" silent with an ear to the driver.

I’ve been following this thread for a while now, so I’ve had the chance to do several tests using my UFO25 and Zu Druid Mk V speakers, which have sensitivity of 101 dB. I’ve used two different sound meters, and three different speaker cable combinations. I did 3 new tests on each channel today.

With the amp and speakers connected by Decware’s ZFocus cable, I averaged 34 dB (range 32-36 dB) at 1 meter from the speaker driver.  This is just above the background level of my room (32-33 dB), which is all the meters registered when I moved back to 6 feet from the driver.   This was with the amp gain set to a level that delivers a baseline 82 dB at my listening position 10 feet away. That 82 dB baseline is actually slightly above my typical comfortable listening level, so if anything the 34 dB hum at 1 meter is slightly louder than I would typically experience.

When I had the UFO25 and speakers wired with the M22759 wire that is the basis for the ZStyx cable, the hum at 1 meter measured just slightly higher at 36 dB, again, just 3 dB above the background noise level in my room. And again, basically inaudible at any distance greater than 6 feet.

Out of curiosity, I did the same test with my 40 wpc ZMA and got 34 dB at 1 meter.

101 dB speakers are, obviously, 4 dB less sensitive than 105 dB speakers, but they would generally be considered pretty sensitive speakers.

I have run the Caintuck Lii 15 OBs (97 dB) with the ZMA, the UFO25, and currently the standard SE84UFO, and never noticed any audible hum at any distance. Indeed, the latter is currently running in our family room and is so dead quiet that I sometimes forget that it’s on for hours or days at a time. The default gain on this amp is set a good bit higher than in my listening room, because the family room is a much larger space and I generally fill it with much louder music.

Take from this what you will, but my own experience doesn’t suggest that there’s an omnipresent hum under normal operating conditions that other Decware listeners just aren’t hearing, even with relatively sensitive speakers.

One final addendum on this: I use my Decware amps with a ZRock2, but without another preamp. As a result, I rarely set the gain on my amps above the 10 o’clock position. As a further experiment, I moved the amp gain to the 3 o’clock position, which someone might use if they were running the amp wide open and using a pre-amp to control the volume. This delivered 39-40 dB of hum at 1 meter, dropping to a still modestly audible 36 dB at 2 meters, and still becoming inaudible at my listening position. This suggests that if you’re running an amp and pre-amp together and encountering a hum at your listening position, modestly lowering the gain on the amp and adding offsetting gain on the preamp might help. That said, given that mk60 experienced hum from his Sarah without a preamp, and with amp set at zero gain, it seems unlikely to help in his situation.

 I just sold a very nice preamp that I made to someone who owns Khorns. Here we go again. In my system little to no hum. In his, hum is there but only when the level is turned up louder than at a normal listening level. He sent the pre back to me and I did a star grounding which helped his hum problem, but was still there. In the end he decided to keep the preamp based on the musicality that it had over some of the other preamps that he owns. Synergy is not easy to get in every system, and as much as I like some of the speakers out there, I see no reason to get above 95dB for sensitivity. you are likely headed for any issue large or small to be noticed above the goal of recreating just music.

Here is a calculator for SPL vs distance. It will overestimate the attentuation (underestimate the SPL at distance) because a real speaker is bigger than a point source, but it should get you in the ballpark.

https://www.wkcgroup.com/tools-room/inverse-square-law-sound-calculator/

Yeah, right, inverse square makes sense. Anyway, the answer to my intended question was obvious my brain was toast last night -- whatever SPL I have at listening position on a 105 dB speaker will simply be 4 db lower if it were coming from a 101 dB speaker.

So, assume the "intrinsic hum" to be a constant level signal coming out of the amp output and into the speaker.

Then:

1. If I took one of my UFO25s that hums out of my La Scalas at ~55 dB, and dropped in Cornwalls or Zu Druid Mk 5s instead of the La Scalas, I'd expect ~51 dB. I think that's still a reasonably noticeable hum?

2. If I take my SEWE300B which is generating ~58 dB of hum out of my La Scalas, and I replace the La Scalas with Steve's 103 dB headwreckers, I'm still at 56 dB of hum, which is not good.

It really seems like what I'm experiencing with my UFO25s is out of line with what others have experienced with their UFO25s, and what I'm experiencing with my SEWE300B is out of line with what Steve is reporting about his experience with his headwreckers. But in my last conversation with Steve about this, he really seemed totally sure that what I'm experiencing is expected. Maybe it's time to bother him again? Although I hate to.

Testing my 300B on the pair of Klipschorns I'm auditioning in another location has been pushed back a week.