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
Set up AA Power 1 mk3 this evening with 2 hrs listening.
First impressions suggest it's a world class amp built to high standards/expectations.
It's difficult to know how the amp will sound after more hrs.but hope deeper bass will be apparent.
Fine acoustial tambers,clean articulate with liquid midrange.Classical sounds best so far and the phono stage is quite good.Also the headphone amp sounds excellent.Tomorrow will give further insight into the amps potential.
Looking foward to hear the amp open up more
Looking forward to your thoughts on the AA Fishing716. I have the MK2 version and love it! My AA had terrible bass when I first got it. It was so bad that I even checked to make sure I didnt reverse the polarity of my speaker cables. After about 50 hours the bass came alive. Now, I think this integrated sounds so great with my SF Auditors. Vocals are tremendous.
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.