Bass leaves after amp warms up?

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.

I agree with Rrog. Its another way to look at what has been said regarding your specific amp/speaker match, ie the amp can drive the speakers, but not OPTIMALLY, which is what is required for good bass and dynamics. The upcoming amp change should be a big step in the right direction. We'll see...

I'm wating for confirmation from my dealer to send new amp.Obviously he would rather not.