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 haven't tried other speakers since I like PSB sound & look..But my second choice would be Joseph audio
I prefer not to advertise,box up & sell synchrony one's unless necessary.
I think ignore comments about learning remote distrupting the sound..too far fecthed

Atmasphere:
For second day in a row after losing the better sound
I shut amp off,came back 1 hr.later & the sound returned to optimal/
If it happens next time I'll consider this a breakthrough of some sort>
NO listener fatigue here
The optimal sound rarely lasts past one CD/LP
The sound goes flat after being dynamic..sometimes before a song ends.
This has been happening for a year w/multiple components,cables & speakers.
But I did stay with Audioquest & PSB not knowing for sure if they were a problem for me.
For the record I have a steady girlfriend that hears when the music changes..
Just the other day said "what happened" like the air out of a balloon.
It's been a while since I used stock power chords but will give it a shot.
Did you ever try a different power cord on the amp?