Bass leaves after amp warms up?

I can't imagine that the amp (unless it's defective) is the problem. Most of the Musical Fidelity stuff is designed and advertised to drive difficult loads. That M6i is supposedly rated at 400 watts into 4 ohms and the poster has said over and over, it never gets warm. Seeing that the poster had the same problem with other amps makes me wonder if we will ever get to the bottom of this.

I'm auditioning Meridian 508.24 cd player with my current setup.It's easy to think this player is just average when dynamics are not working.The sound changed after 15 minutes of being a great player with taut bass to sounding uninspired.
The bottom dropped out before finishing the same track.
Green River/CCR
I'm sure everyone can appreciate how disappointing this phenomema would be.
I will be trying Musical Fidelity M6 250w amp next week.If the problem is not resolved then it's time to part company with PSB.
Which I am very satisfied with when things are working.

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.