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
Can you suggest integrated amps that double it's power into 4 ohms
Not Bryston or Musical Fidelity..I'm not familar with other integrateds
Atmasphere
After enjoying the system sounding great for 2 hrs the bass dried up.
I waited 1 hr.tried again but no improvement.
Turned off amp for 1hr.returned & the presentation was still awful.
I also observed when I change source on the amp it seems to create a break in the circut & the bass leaves soon after.
I've sometime observed after turning a record around/changing channels on a cable box or changing imput buttons on the amp may upset the delicate balance.
Disregard previous post:
Makes no sense!@
But there is no pattern when system sounds optimal:
Could be am or pm
If changing source impfoves thingx temporarily, its possible the source
selector switch on the amp has a problem. Could be dirty or defective.

Ops comment tnat hs is not listening at hign volume would seem to work
against the amp not being abl to drive the speakers well theory, as well as
swirching sources making a temporary difference. Having tne same
problem with two different amps could point to both being underpowered
and heating up over time and negatively affecting sound.