Bass leaves after amp warms up?

To answer your 3rd question
If I turn the amp on and everything sounds good I listen as long as it lasts(about one hour).
I have not tried turning the amp on and waiting an hour to play music.
Also after the full sound leaves I tend to either reconnect speaker cables(which works if I am persistent) or come back later.
It's safe to say I never know if things will be the way I like them with full deep punchy bass or white washed and un emotional.
It's really perculular
Generally the amp plays better cold than warm but sometimes The sound dosen't change for a few hours,but inevitably does.

Foster_9, that's true, some amps are better than others but his amp plays fine for couple of hours and then looses bass. Switching source or adjusting speaker cable can bring back the bass performance instantly.

Fishing716, what about amp's power cord? Is it possible that you used the same cord with both amps? Defective cord would be better news than defective house.

The problem is not related to the amp because I had two different Bryston's prior with the same result. Powerful full sound then it vanishes.
Instruments & vocals no longer ring true
While watching DVD Sountrack like James Bond
The growling & bass effects obviously add enjoyment to the experience.After my system no longer reproduces these effects the movie is no longer worth watching.

I never listen at high volumes.I have a downstairs tenant & don't enjoy music loud