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?"

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.
The amp doesn't feel hot
Can it still be running hot?
Without being familiar with the specific design, we probably can't rule out the possibility that some particular point or points within the unit are getting too hot, even though it seems cool on the outside.

The most meaningful places to touch on the outside are probably the heat sink fins that are at the sides of the amp.

Looking at some photos, it appears that the top cover is held on by 14 screws. If those are removed it will probably come right off.

Regards,
-- Al
I attempted to remove the 14 top screws but don't have the right tool..
Would you know what size allenwench to use or what size/type screws they are.