Bass leaves after amp warms up?

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.

Go to your friendly neighborhood Radio Shack and buy their relatively inexpensive sound level meter and get the Stereophile Test CD and run it on your system and in your room to see what is happening. You can get on the Stereophile web site and order the test CD for quick delivery. The Sound level meter is really nice also. This will give you a very good indication as to what your system and your room is doing without spending tons of money. you will see if there are holes in your bass response for your system/room.