Bass leaves after amp warms up?

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.

Ron, because he said in the original post that the same thing occurred with a different amp.

Best regards,
-- Al

Hmm, why could it not be both amps tried are being overdriven driving the 4 ohm PSBs to the user's satisfaction, especially if listening at very high volumes, and performance deteriorates over time accordingly?

Which Bryston amp was used specifically? Were the limitations over time as pronounced as with the MF integrated? Does the problem mostly occur when listening at higher volume?

The complaint is one I have come to associate with MOSFET output transistors. A small fan can indeed help. The amp may not run all that hot but MOSFETs have a quality where the warmer they get, the more they limit current. So you want to keep them as cool as possible.