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
Certainly any help is appreciated but recordings of the sound differences were sent out and I noticed a difference. I still believe it is somehow related to heat. Either in the amp or the speakers or both. The "bad" sound reminds of an amp that is underpowered or does not have tight control of the bass. My hunch is it is a combination of issues since it never could be replicated consistently.
I'm wondering if OP got his Brystons to run with PSBs or stuck with just headphones as he mentioned in last post a couple weeks back.
I'm thinking if the Grado headphone sound floats OPs boat, BRyston + PSB is a reasonable pairing to try. OR maybe he's just more of a headphones guy. That's a lot cheaper and easier to get right most of the time!!! :^)
I dunno, Mapman...going from 40 inch, 4 driver towers is very different than headphones.