Bass leaves after amp warms up?

Interesting, it is hard to account for all of the reported phenomena. It sound like the poster does not like the inherent sound of his system after all of the gear has properly warmed up. This would mean some kind of system tuning is in order-change in speaker and/or listener placement, room treatment, equipment change, whatever, and perhaps a change in listening habit--e.g., leaving the system on all the time so that it is warmed up from the get go.

Comparison with an amp well suited for the speakers, at least on paper, would seem to be the one thing to try with biggest upside. At a minimum, OP finds out if prime suspect, the amps ability to drive the speakers, is in fact an issue. At best, overall performance may well go up significantly and current problems would be but a memory.

I am hard pressed to imagine how power source would produce the pattern of sound deterioration over time described. Defective speaker wires might be a reasonable explanation. Replacing speaker wires with most anything else, even basic 12-14 gauge zip wire, should be enough to determine if current wires are defective in some way leading to deterioration of sound over time.

The dropout in sound as described seems to fall in line with what one would encounter if one phase of the signal/incoming power feed were to drop out somewhere along the line. Preamp volume control or ac outlet problem possibly? However, I cannot reconcile the fact that it only seems to occur after warmup with my hypothesis.

Ant
When is the electrician coming over?
Best JohnnyR

The speaker cables were already replaced with the same model *audioquest comet biwire

The problem still persists:

I am auditioning different speaker cables tomorrow followed by Electrician to check connections.

I will report back

Thanks to everyone