Bass leaves after amp warms up?

Could it be that you are not hearing less bass just tighter bass as your entire system warms up ?
I prefer to let my system play for about an hour before I do any serious listening. It could be your room or placement of your speakers. Not your amp.
Just a thought.

Most likely your system is on the edge of lean and when it warms up it goes over the edge. Make adjustments in speaker placement and things that make a difference in tonal balance like selecting cables after the system is fully warmed up.

Perhaps,but the bass is taut & punchy while not being bloated before the amp warms up-It sounds right & enjoyable.
The change is dramatic & unmusucal after warming up

I had the same problem with Bryston amp ...

So it would seem that the problem is most likely unrelated to the amp, and that it is related to something else that is occurring during that hour.

Does the same thing happen whether or not the CD player is spinning a CD during that hour?

Does the same thing happen whether you are listening to vinyl or to CDs (I see in your system description that you have a turntable in your rig)?

Does the same thing happen if you are not playing music during that hour, so that the speaker drivers are not being warmed up/loosened up during that time?

Regards,
-- Al

Okay fish, you're a professional musician. You've dismissed all ideas and suggestions out of hand. I guess you're on your own. Good luck and I hope the bass just mysteriously reappears for you.