Thanks Beelzebum and Robin for getting back to me:
Robin: the initial silent amp problem seemed fixed when I unplugged one of the two speakers (suddenly there was sound, the other speaker worked fine). Once I unscrewed the non-working 1/4″ plug, then the non-working speaker worked too so it definitely seems like a shorted plug was the problem.
Beelzebum: shrink wrapping the wires separately before screwing the 1/4″ plug together is a great tip! Thanks for that.
I’ve wiggled the power cord and other wires, torqued the chassis and poked components. All other connections seem good. It was only wiggling wires between the one non-working speaker and it’s 1/4″ plug that made things intermitent (or screwing that plug together).
One check I still haven’t done is for shorts in the impedence switch itself. I’m thinking that I need to remove the two wires from the OT before I can use my multimeter to check open/closed circuits when the switch is flipped? Right now resistance readings on the switch tabs just fluctuate, then return to what they were when I flip it. I would have expected the “off side” impedance switch tabs to go to high resistance when the other side was selected. Hrmmm.