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
Since having the PSB all 3 amps have given the same results with exact same symptoms.
Irony is I like the PSB Synchrony sound
I borrowed Thiels for a very brief period and found them to be bright.Bass not being consistant was inconclusive.

Currently just listened 2 complete LP's without losing the bass using just the right speaker connected.
I don't know why that is
I'm currently monitering this effect & duration
Currently just listened 2 complete LP's without losing the bass using just the right speaker connected. I don't know why that is.
Connecting just one speaker will certainly make life easier for the amplifier, and reduce the internal temperatures that it will reach, compared to playing music into two speakers at a similar volume setting.

Regards
-- Al
If the problem occurs in both speakers at same time when it occurs and does not with one, that would tend to indicate that amp/switch/control defect is less likely, but still possible. Definitely supports the amp is overworked due to load and getting too hot theory.
BTW testing with just one speaker in lieu of others to test with was a good call! Gotta remember that one.