Bass leaves after amp warms up?

No All the connections are always tight
I have spades and the speaker cables are (2nd) pair of AudioQuest comet biwire 6 ft.
No heat on either end of the cables

Amp could be defective.

Not likely though that two amps would have the same inherent defect though.

Switching source has an effect I recall. COuld be defective source control switch. COuld be defective volume control if volume is adjusted at all in test cases.

Fishing,

Any difference at all in when the problem occurs between old amp and new? If so, equating the two to produce exactly the same problem may be confusing the issue.

My guess at this point is it is some combination of these that is causing the problem.

1) defect in the amp that affects performance over time
2) amp is having problem with speaker load
3) patterns of human hearing as identified above.

Trying a different amp that is up to the task on paper or better as measured on the bench would address both 1 and 2.

Not much one can do about 3 except be aware when doing listening tests.