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?
Hmm, well I thought i read that Yamaha model to be Class D, but maybe not.

Maybe this amp will do better than the last one. I tend to like most Yamaha amps I have heard, but have not heard that one. I hope it works out.
Fishing ... Class D as compared to class A, AB amps....
Can anyone recommend Yamaha paired w/PSB Synchrony one's or is Musical Fidelity pre/prx much better choice?
Don't know, but again the issue of of output power comes up. You need an amp that doubles down it's power from an 8 ohm load to 4 ohms. The Yamaha does not. (Remember the impedance curve of the Synch One drops below 4 ohms).
That's the solution to drive those speakers IMO.

Look for this topic in the earlier comments. Go looking for Almarg's postings....and Dan Wright's info.