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
I gave more ventalation by removing the shelf above the amp..
The Bass still left after 1h 20 minutes.
I was listening to "Let it be" & that was my parting words after the sound changed.
If everything has been swapped out and the one constant still in the system is the speakers ? Then i would be willing to wager it is the speakers ! No ?
Bass returned after waiting 30 minutes.
After about 30 minutes of sounding good it disappated.
Surely something causes deep bass to come & go as it wishes.
I have(3)ideas to try this week:
Electrician is going to check connections
I am borrowing different speaker cables
May put up $1745(demo)& try ps audio p-3 power conditioner with return previlage.
Yes. If I'm not mistaken, the PS Audio P-3 connects one lead to the right temple and the other to the left. Be very careful in plugging in the ground:)