Bass leaves after amp warms up?

No way! Are the Bryston integrateds as muscular as a separate Bryston power amps. Call the factory and ask the techs. I can't go back to read all the steps taken to isolate the problem, but I assume working backwards you eliminated your source components (i.e., CDP and phono, as applicable), cables, etc.

If so, that could mean, as has been surmised above, the "true" impedance of your speakers may be lower than the nominal 4 ohms I seem to recall being mentioned. Actual versus "nominal" reported impedance stats can vary quite significantly and often do. Very few amps "like" driving into those kinds of loads. If a monster Bryston doesn't work, I think you go back to "Zeros" and see if that helps.

Btw, just an "fyi" I picked up some time ago when talking to Kal at ARC. He mentioned that ARC tube amps were used to drive Wilsons which have notoriously low impedance stats. Just an fyi.

Otherwise, as Al says, you have to consider the only remaining possibility, no matter how improbable: Poltergeists.

I'm going to see if your PSB speakers were bench tested by any of the reviewers. If so, I'll be back.

I asked mike at Bryston when considering Preamp BP17 & power amp 2.5BSST
He said they were the same as B135sst integrated in seperate boxes

Ok Fish..., here's what I got off the web.

HT Lab Test (12/07): The Synchrony One's listening-window response (a five-point average of axial and +/–15-degree horizontal and vertical responses) measures +1.18/–2.24 decibels from 200 hertz to 10 kilohertz. The –3-dB point is at 56 Hz, and the –6-dB point is at 50 Hz. Impedance reaches a minimum of 3.32 ohms at 70 Hz and a phase angle of –37.55 degrees at 27 Hz.

SoundStage (1/08):

http://www.soundstagenetwork.com/measurements/speakers/psb_synchrony_one/

The impedance magnitude and phase angle graphs are absolutely amazing!! I've never seen flatter. Problem is impedance over large swathes of frequencies is unbelievably low, especially in the bass frequencies.

Al and Ralph -- take a look at the graphs at the URL above. I'd like to read your thoughts about whether a solid state amp, or any amp for that matter, would get indigestion from those impedances. Fish ..., if Al and Ralph come back expressing concern, the Zeros are seriously looking like the solution.

BIF

Fishing, here is an article about the other conditioner that was referenced in the AudioPrism review. It’s long, but read carefully @ Listening Up… the Game is Afoot. The last 3 sentences of the 1st paragraph seem to somewhat describe your problem. The next paragraph talks about positioning the conditioner, and further down shows a picture of a fairly extensive setup.

http://www.soundstage.com/noisy15.htm

I may be ”grasping at straws”, but just trying to help. The best way to determine if the AudioPrism is causing a problem is to take it out completely. Just use something like a heavy duty outlet strip to get enough plugs for everything, and you probably will notice some degradation of sound, but play the system to see if the problem occurs. If it does, we’ll just keep looking for other ideas. At least, this is something simple to do, and I don't think it is something you have tried.

I've tried power strip recommended by Naim/one by emotiva/wall outlet & Audio prism.
The best sound was with the Foundation II.
Bass issues were never stabilized by anyone of them.