Bass leaves after amp warms up?

One thing to add is that a less than optimal speaker/amp pairing does not necessarily mean that either device is faulty or defective. Just that they are not the best match for each other electronically.

Given that, depending on how long you have owned the amp or speakers, the best one might expect from a dealer is perhaps some trade-in value towards an upgrade. That's fine. Live and learn! If it has not been too long, a good dealer might offer full trade value, but even that is not a guarantee.

So if you want to move forward, find out what your dealer can and will do for you, but consider other buying options as well, including buying used to help minimize the loss when you try something that may or may not work out, which is often the case when one has discriminating ears.

I can't assume a dedicated power amp will solve the problem till I try.

What was the most recent change to your system?

"'I can't assume a dedicated power amp will solve the problem till I try."

It's an assumption until you try but based on sound supporting evidence that the amp is not the best amp for those nominal 4 ohm load speakers. Other than this, pretty much all else is pure speculation at this point. So address the known issue first. It will persist no matter what else you try or do otherwise.

"What was the most recent change to your system"?
Nothing,just a different cd player

Musical Fidelity integrated amp & phono preamp
PSB Synchrony Speakers
Rega P5 TT w/dynavector cart
Audioquest cables