Bass leaves after amp warms up?

Is there any correlation for time of day? Maybe there is a difference in the power quality feed into your premise based on something else happening around your local grid? A long shot I know.

Since you have controlled for so many elements, the most likely culprit to suspect is YOU. I would set up a controlled blind test with an audiophile friend so you would listen to the system for 10-minutes on at least 4 separate occasions without knowing if the system was just turned on or if it had been on playing music for an hour or more. If this proves the effect is not imagined then I would sell all my equipment and move.

Certainly any help is appreciated but recordings of the sound differences were sent out and I noticed a difference. I still believe it is somehow related to heat. Either in the amp or the speakers or both. The "bad" sound reminds of an amp that is underpowered or does not have tight control of the bass. My hunch is it is a combination of issues since it never could be replicated consistently.

It is my understanding the OP has moved on.

I'm wondering if OP got his Brystons to run with PSBs or stuck with just headphones as he mentioned in last post a couple weeks back.

I'm thinking if the Grado headphone sound floats OPs boat, BRyston + PSB is a reasonable pairing to try. OR maybe he's just more of a headphones guy. That's a lot cheaper and easier to get right most of the time!!! :^)