Bass leaves after amp warms up?

I just got off the phone with Musical Fidelity..
I was assured the M6i is more than capable of handling PSB Synchrony one speakers.

I'm not sure that an issue with eletrical power supply would be consistent with the pattern OP describes for sound deteriorating over time.

The amp would appear to NOT be up to the task of driver the PSBs OPTIMALLY given its bench measurements Al identified. That is a clear issue that would account for deficiencies in bass and dynamics.

Find an amp to try that doubles down mostly from 8 to 4 ohms. You might not even need more than 100-200 w/ch to do a good job assuming you do not listen at high volumes. If you do, then throw the kitchen sink at those puppies. Modern Class D amps make it more practical than ever to do this when needed.

Check power rating into 4 ohms to see if double or close to double from 8 ohm. That is a good indicator. Published bench measurements in reviews to confirm is best to be sure.

I could recommend some to try as could others I am sure if a price point were known?

"I just got off the phone with Musical Fidelity..
I was assured the M6i is more than capable of handling PSB Synchrony one speakers."

This is a subjective statement. Most listeners out there might be more than satisfied. There is no reason for MF to say otherwise. But there are doubts based on published specs and bench measurements that should matter to a very discriminating listener, which OP appears to be. So there is a CLEAR red flag worth consideration.

I've heard the PSBs sound very good off 100w/ch Rowland Class D switching amp. That would be a good place to start as a reference. I truly believe a good modern Class D amp with good power supply that can mostly double down to 4 ohms can solve OPs problem in a relatively inexpensive and very compact package.

Rowland 100 w amp for reference.