Bass leaves after amp warms up?

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.

Fishing716 mentioned also on another thread that switching sources can bring back the bass. Same happens when he "adjusts" speaker cables. It would imply bad connection somewhere. It is not amp because he tried two, not source since he switched them, not speaker cables because he had this with spades, bananas and direct wiring. I also believe he tried different speakers? It is not line voltage since he stated it is 120V. It is not outlet because he tried few.

Is it related to equipment or room/building? It might be bad connection somewhere in the building (defective breaker, loose screw in the panel etc.) I would set-up the system with small speakers and just CDP to verify that it is still happening and then would get it to my another house.

I considered defective jumper from pre-out to power-in but Bryston has switch.

Good call. Bad connections happen and could easily produce the effects observed.

If the problem occurs at lower volumes as well as high, that would better indicate that temperature inside amp over time when driving speaker load is not a factor as well.