Bass leaves after amp warms up?

Can you take the cover off and test as Al suggested?

If that helps, a good external + quiet cooling fan might help. Or if you have any fan that could be applied, just try that. ANything to help lower the internal temperature and see what happens.

BTW, if you can prove via testing that cooling the amp helps the sound quality, that would add leverage for you with your dealer and MF to get them to perhaps offer a trade or other solution to help you get better results with the PSBs. Especially since MF has told you what you have should be fine.

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.