Bass leaves after amp warms up?

I would not use any power conditioner on power amp or integrated (which includes power amp) unless it is rated to deliver enough power and current. Use it with source devices and pre-amp but not device that provides power (integrated in this case). Most power conditioners that are suitable for use with high power amps are somewhat expensive due to build quality and power delivery capability needed.

I would keep power conditioner out of the equation with the integrated for now. Once issue is resolved, then you might try it and see. IT could well be contributing to the problem. Wish I knew it was in use earlier......

Here is the info on the Autoformer Zeros Devices

I agree that these appear to be a viable solution for providing your amp a load that it is more suited to drive. Resulting sound should be different, but assuming the amp/speaker load match is the issue, which seems the most likely still, this should resolve any issues with not enough gas for the bass at reasonable volumes.

Should be different for the better.

I'm pretty sure the BRyston amp you would need to drive the PSB load effectively is not the same as what you would find in any typical integrated amp, but have not done the research on Bryston specs to say for sure. I would not equate any high power integrated to top notch similar powered separate amps without having the numbers in front of me that establish that.

I would keep power conditioners out of the equation period. The symptoms occur with system warm up. If anything the OP should be backtracking to find out where it all started.

"I would keep power conditioners out of the equation period. "

Agree. Once the issue with deteriorating bass/dynamics is resolved, then add it in gradually perhaps and see what might be had further from there. Power amp would be last thing to use with a conditioner and should be totally optional at that point.