Ok I wound up trying one more thing, I swapped in a 5AR4 Rectifier to see if that would change things (up to now I had just tried a different 5U4G). Unfortunately, this still had the same hum at the same level.

To see if this was inherent to my room/power for a tube amp, I hooked up a Reisong A10, which is another low power SET amp so I figure it's a decent comp on just baseline tube-amp-in-my-room expectations. It was nearly silent, with hum only audible with my ear next to the speaker (measured with my phone's dB app at 45dB, compared to 55dB for the SE84UFO25... the SE84UFO25 measured 9mV with a multimeter). Unfortunately, the SE84UFO25's hum is audible where I sit, so I think my best bet is to shoot Steve and email at this point, see if there's anything else to try, or if I need to send it back (if I couldn't hear it where I sit, I'd live with it, but it is actively annoying, so I'm gonna try everything to chase it down).

No, I emailed Steve and wound up sending it back for inspection/fixing/adding some volume knobs (I ordered it without volume knobs because I also ordered a CSP325, thought about it for the 18 months I was in the wait list, never changed my mind, and then on delivery was like "shoulda gotten the volume knobs")... I'll let you know if he finds anything!

Alright, I think I've got as close as I can get to solving this, here's where I'm at:

I talked to Steve last week, and he said what I was hearing was the power supply ripple. This is a bummer, I've got other tube amps that are quieter at idle, but it doesn't seem there's much that can be done here (if there is, I'm certainly open to suggestions!).

However, I've got some good news too: through the course of all my testing, I did notice that the transformers were quietly humming, which was essentially being amplified by the case (e.g. putting your ear next to the case, you could hear it much louder than just putting your ear next to the transformer). So, I ordered some small rubber washers from amazon, and very carefully loosened the mounting bolts on the transformer and slid them under (I cut a notch in the washers so I didn't need to fully remove the screw). This worked surprisingly well (well, it was kind of annoying to do it, there's a bunch of threadlocker and you want to... not break stuff/shock yourself), the noise from the transformer resonating is greatly diminished, Decware should consider just installing transformers with washers like this to begin with. The hum is still there, but just barely, I really have to strain at this point, and if I ever move to a larger room, I doubt I'd hear it at all.

Yeah, I basically agree, it would be nice if at these prices I wasn't like "well I think I can live with this"... but it is what it is... part of it is I waited 18 months for these so I really want them to work out... a bigger part too is that they really do sound amazing, I haven't heard anything that sounds like these do. I suppose the other piece too is if it really does wind up driving me nuts, I can always sell it and make someone's day and let them skip the wait line!

Yeah all signs point to it being power supply ripple (including when I talked with Steve). Wish it wasn't there, but as far as I can tell, there's nothing really that can be done. My listening room for now is small so I'm really close to the speakers (probably about 4-5 feet), a few more feet back and I doubt I'd hear it at all. Still on the fence about selling, but nothing really sounds like this amp, so I want to give it a shot in a different room before I make a final decision!

Chris,

I don't think that the amp would be capable of producing anything without tubes. Maybe you meant replacing ech tube one at a time?
The shorting plugs are something that I used just yesterday during a preamp repair. Sure enough without the plugs in the input selected there was a hum. Star grounding alleviated that but only when a component was connected.
Hum can come from various sources or reasons. Power supply ripple is one that may point to a rectifier tube, but if not, then there is a problem that needs to be resolved. Trouble is, there are quite a few amp designs out there, and each one has its shortcomings. Some amps are prone to hum more than others.

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Is that hum through the speakers, or mechanical hum from the amp itself? Without power tubes I'm pretty sure there is no audio output from the amp, the speaker outputs is connected to an empty tube socket through the output transformer. But you could be getting mechanical hum from e.g. the power transformer. If you are getting hum through the speaker my guess is you are picking up some kind of noise, possibly from the speaker cables esp. if they are long.

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Interesting, that seems like a pretty basic design consideration.