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
I have analyzed and listened to "good" and "bad" recordings which Fishing created of Dawn's "Candida," in similar fashion to what I previously reported doing with a recording of "Respect Yourself."

It appears that the recordings of "Candida" were done with reasonable level settings, and with ALC and peak limiting properly disabled. I found that the "good" recording was about 0.4 or 0.5 db lower in both peak and rms volumes than the "bad" recording. Essentially the same 0.4 or 0.5 db difference existed between the volumes the two recordings when frequencies above 300 Hz were filtered out, and also between the volumes of the two recordings when frequencies below 300 Hz were filtered out.

The sound quality was generally very poor in both recordings. I suspect that the major reason for that was the low quality of the recorder's built-in mic. The single built-in mic, of course, also resulted in the sound being monophonic. Nevertheless, I sensed a VERY slight loss of definition in the bass on the "bad" recording. Not looseness, not weakness, just an almost unnoticeable loss of definition. Presumably the difference would have been much more apparent if the recorder and its built-in mic had not been in the loop.

Fishing also provided yesterday a "good" and "bad" re-recording of "Respect Yourself." I did not listen to those, but my software revealed nothing more than about 0.2 or 0.3 db differences in volume, frequency content above 300 Hz, and frequency content below 300 Hz. As with "Candida," the "bad" recording was the (VERY slightly) louder of the two.

Finally, today he provided a second set of re-recordings of "Respect Yourself." However, the recording level on these apparently was set too high, and the waveforms were clipped in numerous places, so I didn't do anything further with them.

FWIW.

Regards,
-- Al
Fishing's girlfriend hears the same differences. So there should be no doubt that its a real phenomena.
"04-20-13: Csontos
Quiescent current is higher when the amp is cold and decreases as it heats up whether the outputs are mosfets or bi-polar. But more so with mosfets than with bi-polar outputs.
Csontos (Threads | Answers | This Thread)"

Not so with the two powerhouse ME850's I have here, both are at 20mv accross the emiter resistors 12 per channel when cold and by 20mins they have climbed and stablized to 50mV. My bass is ultra tight but 2 dimential when cold then it looses precieved tightness for a more 3 dimentail bass when warm. I believe this is what's happening to the poster. Or his supply is sagging as the bias climbs if it goes up like mine as it's warming up.

Cheers George
Hi George. Yes, I will observe it climbing from turn-on to warm-up also. By 'cold' I meant not having yet played music. But as the amp is taxed, current will fall off ime.