It was a contact problem.
I got a can of CRC QD Electronic Cleaner (Quick Drying Formula), powered off the monitor, sprayed it into each switch and pot, played with each switch and pot about 10 times, put it outside to dry for about 20 minutes, turned it on, and it works fine. Repeated with the other monitor with the same results.
I may still need to contact your friend to work on one of the speakers, as one tweeter sounds different from the other, but for now I'm up and running again. Thanks, Lights.