Bass leaves after amp warms up?

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

Fishing, also I am still wondering if volume of listening is a factor. How loud do/can you listen? Does the problem occur mostly at lower volume or high, or does volume not matter, same results?

If associated with lower volumes as well as high, a connection issue becomes more likely. COuld be a power cord conenction even as Kijanki suggests.

As you describe it, I would expect teh problem to be most noticeable at higher volumes where bass "pressurizes" the room more normally, but if clearly noticeable at lower volumes still, amps ability to drive the speakers optimally may not be the prime issue to address first and then see.

I never ever listen to music loud

If you had the same problem with the Bryston amp it's very unlikely a power amp issue. Sounds like a power issue. Either line/conditioner or pc. Have you tried switching power cords? Maybe a defective conditioner? Voltage flucuation?