I guess it just depends....

Given a certain amp power and speaker efficiency, I find room size and loudest listening preferences can both make pretty big differences with lower power amps...

My "listening room" is the main part of a relatively open space that segues into other spaces with turns and big openings. But I think it is a pretty contiguous volume considering the stereo placement. It adds up to somewhere over 5000 cu ft. With similarly rated HR-1 speakers, and a Torii IV, for late night louder listening, my most serious listening mode and loudest play, it can be on the edge. I am not comfortable with average dB measurements over 75-80 (preferring lower averages), with transient peaks on rare occasion up nearer 85-90, and for this, the way I am setup, I need a Torii MKIV or more power.

But.... I do tend to prefer milder tubes than stock, and I love the sound this way, but it does not sound like I am getting the fullest available power. Also, I do tend to run a number of pre stages between DAC and amp. And especially with more than one, adding stages seems to progressively concentrate the signal enough to cause distortion in the Torii at a little lower loudness levels.

I don't know exactly what is going on with "stacking" stages as Steve called it, but with "gains" of each high enough to effect the overall sound positively, it seems each adjusts the signal progressively, and this can add up.

I find higher DAC signal out voltages can be part of this, so lowering player software settings with a higher output DAC can be one of my gain adjustment choices. Pre stages are all about enhancing the musical beauty of the signal to me, and can do this with more complexity in succession. I 1st get them all to sound most complete individually, each doing its thing with the sweetest, most transparent sound, and then tune to balance them together musically. So each is above unity gain, contributing its particular flavors of natural sounding enhancement, and each is changing the signal. I imagine it something like this: each pre stage adding some lucidity, dynamics, weight, resolution, density, spatial and harmonic complexity... in their own ways... the signal clarity, space, density and hit adds up, and this seems to lower the max volume of the amp some.

Just pointing to some of the things that I have noticed to contribute if the system/room is closer to an edge power wise for max use.

I wish Steve or someone who knows would explain the tech of this progressive stage use, but these are my observations.

Of course my room, system, but I have tried a Rachel and Taboo here and those two did not have enough power for these speakers, and this specific setup and needs. And, for me, even the Torii IV, can be on the edge depending on my gear setup.

I also suspect that the differences between Single Ended and Push-Pull with Steve's designs continue to narrow as his design evolves. I know with my pretty intensely upgraded Torii IV, I hear the fullness and power of Steve's push-pull, but also find the spacial lucidity and speed, not exactly the same, but comparing pretty well with my recollections of an 34SE I had. I actually like the complexity, space and speed from this modified Torii better (but I have been the one tuning it)...and I need the power here.

Anyway, I have not heard the MKV, but, by the looks of the design changes, I imagine it is quite fast and lucid. I love how Steve's love of this world starts with his own search for beauty, keeping him searching and creative, and  as he learns and discovers, he continues to improve Decware.

That said, I would love to have a system with more efficient speakers so that I could explore SE more. I just love this one so much, I have not dug in.

I can imagine the Rachel being more between SE84s and earlier Torii MK? models from comparing the Rachel to the Taboo, which no doubt is different too... but using similar tubes as the SE 84.

As you remind me, I was talking about my adjusted Rachel sound. And thinking back, I guess all in all, I found the Rachel a little warm/slow and soft for me with stock tubes. And lacking in power with my 94 dB MG944s in this room on enough recordings to matter, I learned this by pushing the sense of power with tubes. I shifted to 6N2Ps, KT77s, and a GZ34 over a little while.... everything pushing the amp a little more and with some of these choices being more lucid and faster tubes, this contributed to part of that sense of being "louder." It was after doing this for "power" sense I realized I preferred this sort of maxed open/fast/articulate sound from this amp, at least since here it did those things without too much hardness. Long time now, it is a little vague, but my recollections.

Whereas, like I mentioned, though the MKIV was originally less forceful feeling than the MKIII, and the MKIV even less so now with modifications, I still prefer milder tubes for this very reason ... Max power tubes in my Toriis just concentrate the signal too much for me in general, so on the full and forceful sides for my ultimate preferences. But luckily there are so many ways to relax this, and sweet sounding very high quality tubes that are milder in just-so ways can open the focussed density and fullness into faster bass, more open space, speed, lucidity, inner and fine detail, sense of complexity, etc, etc.

But as we have talked about, so much of this common question of power comes down to speakers, space, and listening needs.

I don't rattle any windows here, preferring roughly 70s averages in general for serious listening from the seat. But I wonder if the dB averages necessarily tell the whole story with the impressive density, resolution and dynamics from these amps... you know, the thing where you can listen pretty quiet and get a lot of pleasure because the sound is so dense and complete. And with optimized signal density and dynamics, does it take a little more power to get the same volume? And if so, with the slightly lower volume more complete and resolving does it matter... just imagining...

Whatever.... the signal to me at average Db levels mainly in the 70s, and with dynamic peaks in 80's, in general this is plenty loud for me at these measured levels, but also near max before distorting some transient hits in this room and the ways I usually have it set up.

Here, for me, the acid test has been a pain over several attempts. So though I agree, it is always best to test in the room and system, I think it is also good to make calculated guesses based on our knowledge of these important parameters, and then decide... If I am near the edge, is it worth it to me to try an amp and possibly have to send it back, and then wait for another ? Tricky some times.

Hey Lon, do you have a position on monoblocked UFOs (6w) vs. single UFOs (2.?w)?

I sometimes wonder if just going up the sensitivity ladder on speakers would be enough to make monoblocked Decware SETs unnecessary. I'm sure there are many wonderful speakers below 93 dB sensitivity or so, but I don't see why I can't find something to like with higher sensitivity. Just makes it easier to choose, since the set of options is limited.

I'm not going to have a collection of speakers I rotate...

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I'm a numbers guy so I would look at it like this: monoblocking UFOs buys you  3, maybe 4 dB, so e.g. 88 dB speakers would look like 91 or 92, 91 dB speakers would look like 94 or 95 dB and so on and so on. Which is another way of saying you are exactly right. You can get to the same place with either more watts or more efficient speakers or some of both.

Now, there is another reason to go with monoblocked UFOs, and it the reason I chose them: not for the watts (although they are a nice bonus) but to configure them as balanced monoblocks. This goes back to the discussion about whether or not you need a ZBIT for your balanced DAC, and my solution has always been to maintain the balanced signal all the way to the speakers, rather than either using the RCA output from the DAC or a ZBIT to convert balanced to single ended.