Bass leaves after amp warms up?

Or even just with the top cover off. Better yet, two sets of recordings should be done; with and without added cooling. This could end up being a very fruitful case study on due diligence by ss amp manufacturers. No wonder there's separate tube and ss camps. Tubes and mosfets do after all share some similarities.

I Listened to my system all morning and discovered what is changing in addition to the bass is overall dynamics & timing..When the sound changes the music seems to sag and it's hard to get into the music.
Like getting on an escalator and "riding" with the music/The second version blocks or prevents following the music in the same way.
Like being "out of sync"
So Weird

That's what happens when the midbass is down in level.

How does midbass go down in level?

It's called tonal balance and it's all about setting up a system to your liking. I mentioned this a couple of times on this thread and I think a couple of folks agree with me when I say your system is leaning out as it warms up. For the majority of systems this is a good thing because the system resolution is increased, distortion goes down and the system opens up. In your case you are already there when the system is cold and as it warms up the balance becomes too lean. As I mentioned in an earlier post on this thread, I have had a similar experience several years ago. I corrected the problem by changing the speakers. So, the system should not sound it's best when it is first turned on and improve as it warms up. In my opinion you need to change one or more components to get the desired sound. This is just my opinion.