Bass leaves after amp warms up?


I don't understand-after my Musical Fidelity M6i amp warms up for about an hour I notice the deep bass & kick drum aren't the same.
They sound less musical with loss of weight/depth.The notes are there but the moving of air have left.Sound is has much less impact and boreing.
I had the same problem with Bryston amp so there is no defect with amps nor with the rest of my equipment/
PSB Synchrony one speakers,AQ cables,Bryston CD Player.
My question has anyone heard similar & is there a plausable reason?
fishing716
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
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.