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
Yes but why is the amp running hot if they sell them by the thousands and no report of this problem?
"I'd really like to know what happens with cover off and/or better ventilation"

Guess it's in my best interest to take the cover off & try it.
I'll report back
"Yes but why is the amp running hot if they sell them by the thousands and no report of this problem?"

Each system is unique. You have different speakers than the next guy, diff cables, different power, etc.
There have been other threads where an amp was being pushed too hard and was running hot. One of the troubleshooting tools is to take the cover off.