Bass leaves after amp warms up?

Is there any conceivable way that. after some playing time, one speaker's crossover is having a major phase shift in the bass that is creating this problem?

to "Drubin". IME designing and building custom speakers I have found that excessive heat (in the voice coil, inductors and resistors) will not affect woofer phase nearly as much as the total Qts of a ported speaker. It can change enough to make the bass port tuning looser. However I have been exploring that possibility with the OP and his responces do not suggest that he is getting the speakers nearly hot enough plus he is using the tuning plugs in the lower woofer ports which would minimize any changes in the speakers Qts.

"What was the most recent change to your system before the missing bass syndrome?"

The reason I asked this question is because it is easier to backtrack than moving forward and continuing to try new equipment. There is some point where it all began. So, what did you change last? This should give you a clue of where the problem is.

I changed amp but previous two gave same result
I changed from PSB Synchrony II to one with same result
It effects all sources cd,tt,video

I added a phono preamp last but took that out with same results

So did you ever NOT have this result? If so, what was the system/environment then?