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
Thanks for your response Tboooe

The bass sounds weak & assumed I'm having same problems as before.Hope I hear improvement before 15 day trial period ends.
Fishing, break-in does not affect bass in amplifiers so much. It does in speakers though.

If you have not heard the bass come around in a week we can safely assume (as we did before) that it is not the amplifier at fault.
"If you have not heard the bass come around in a week we can safely assume (as we did before) that it is not the amplifier at fault."

I have not read anything that indicates this amplifier will be the nth degree in terms of extended and dynamic bass like a good doubling down power amp would be with the PSBs (still those, right?), but it should be quite good otherwise as best I can tell. Amps that an better this one in every way should be much fewer and far between though I would say.