Bass leaves after amp warms up?

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.

You might also try a different pair of easy to drive speakers as a test and see what happens there.

The problems are exactly the same with all 3 amps
(2)bryston & Musical Fidelity
Deep bass for about 1 hr
Longest it lasted was 3 days then sound changes with loss of defination.

I seem to notice it's easier to retrieve deep bass with just one speaker connected rather than both.
(Either side)
Sill fades away after apx. 1 hr

Do you have any other speakers around? What happens with them? Results would help determine if speaker/amp match or defect in amp is an issue.

I only have these speakers

The bass is firm & lasts longer with just one speaker connected.Eventually leaves but right now it is working for about 30 minutes
Listening to Chicago V on Rega P5 TT w/dynavector.
I would keep this sound w/one speaker & loss of half information.
But soon it will leave.
I am monitering the situation