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, I should have stated the cautions that the amplifier should be unplugged from the AC when the cover is being removed, and nothing internal to it should be touched whether it is plugged in or not.

Regards,
-- Al
I would not have plugged it in or touched inside.
Was unable to find a tool to try instead took shelf off above the amp for more ventalation
If some or All output transistor without good contact to their heat-sink, the output transistor will run very hot. (The amp dosen't feel hot
Can it still be running hot?)
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.