Bass leaves after amp warms up?

Yes this phenomena happens with Vinyl or CD source
I believe wheather it's spinning or not the sound changes after the amp is on about an hour or less.
Also if I change source from CD to vinyl the sound frequently changes for the worse faster.As if I've distrubted the circut

All amps are no perfectly efficient and produce heat as a result. If the heat produced is not dissipated and temperatures rise to much accordingly, I would expect sound quality can be affected.

CAn't say why this might be occurring in the OPs case, but better ventilation around the amp, maybe even use of external fans, might help.

Based on specs the amp should be largely up to the task of driving the PSBs, but driving most speakers to very high volumes might tax most any smaller integrated amp I have seen to some extent.

I had a Musical Fidelity A3CR power amp that I believe was heavily biased to Class A and was of modest size yet quite heavy and tended to run quite hot accordingly. Might be a similar case here. I moved to a very efficient Class D amp that never breaks a sweat regardless of volume with my lesser efficient OHM speakers. Bass and dynamics is never an issue these days at any volume.

What about my third question? If, after the system has been unused for a period of time that is typical of your listening habits, you turn the amp on, play no music for the first hour, and then start to play music, what does it sound like?

Regards,
-- Al

One red flag I see that might help account for what the OP hears is that his PSB speakers show 4 ohm nominal impedance and the MF amp specs indicate power into 8 ohms only. Together, that might account for what the OP hears. Amp specs or bench measurements in a case involving a 4 ohm nominal impedance load would be more informative.

BTW those PSBs are one of my favorite speakers.

I have heard them run quite nicely in a smaller room and at fulfilling volume off of 80-100 W Rogue tube amp or slightly higher powered Rowland Class D amp.

I did not notice any dynamic compression at the time I heard, however audition was limited.

I would be wary of most any smaller integrated to match this performance though. Tight physical proximity of power amp and pre-amp in the same box creates real limits on what can be achieved compared to separates.

I would want to run those PSBs of the best and beefiest standalone amp I could find if it were me, to help assure OPTIMAL, not just good performance.

Al, I am curious why you think the problem is not related to the amp.