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
You're right I haven't made any progress
The problem persists

I didn't expect to have a problem with such high end equipment.
Apparently what you say about the amp seems to be happening:

"if the power supply is not up to the task it will sag on bass transients more when the amp is warmed up than when it's cold. As the bias is asking for more current when it's warm than when it's cold".

I should either change speakers or amp right?
"I didn't expect to have a problem with such high end equipment."

Now that's funny.

It is my understanding you have tried more than one amplifier. So, you think the power supply is sagging on all of the amplifiers you tried with identical results?
I'm betting the SPL test when it happens will be inconclusive.

Fishing, do you have a CD with frequency-specific tones you can use, like one of the old Stereophile Test CDs? One-third octave warble tones would probably be good and you want individually measure enough of them to cover the range from deep bass up through the midrange.