Bass leaves after amp warms up?

IMO the amp should not dictate what speakers you own. It's the other way around. I refuse to believe that there is not an amp out there that will do a great job of driving the PSB's without losing some bass control as it warms up. In reviews that I've read the Musical Fidelity's are an excellent product but they are not known to have the best bass control. Each of us have a different set of priorities when we put together our systems. After listening to both of the "good and bad" recorded tracks, IMO, "Fishing" needs an amp that maintains very tight control of the bass. Hopefully the larger Musical Fidelity will keep all of the sonic properties of the smaller but maintain better control of the bass by virtue of it's higher cuurent capability and bigger probably "stiffer" power supply. Only time will tell.

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