Bass leaves after amp warms up?

Agree...take the cover off, it's the only thing you haven't tried. Put your trust in Al and Mapman.

I don't think I'm good at taking the cover off.
I'll have a look at it

The fact that the system seems to work while running only one channel is a powerful clue.

The amps themselves are ruled out, as 3 amps have had the same behavior. Also ruled out is hearing and the speakers. The preamp and source/s are still under suspicion.

This has something to do with warmup, but I suspect not in the amp or the speaker. My suspicion right now is a defective power cord or wall outlet that heats up with enough current through it such that it eventually starves whatever amp is plugged into it. My suspicon leans this way because if it was a preamp it would not care if only one channel was running.

If the wall outlet, or either end of the power cord for the amp gets warm over time then I would think that this hypothesis is confirmed.

If the power cords and wall outlet seem OK, then the preamp and sources are under suspicion.

Ralph, note that all three of the "amps" that have been tried are integrated amplifiers. There is no preamp, although presumably there is a phono stage that is not listed in Fishing's system description. But the phono stage and turntable setup would appear to be ruled out as suspects, because the same problem occurs when the CD source is used.

Best regards,
-- Al

I use Musical Fidelity M1 vinl phono stage.
Listed uder my system