Wow! Congratulations! Can’t wait to hear what the solution was.
Jim, sorry for not responding to your post of yesterday morning sooner. It’s a busy weekend here. Your points were well taken, IMO, and as you’ve seen Joe subsequently confirmed that continuity existed per your suggestion.
I would highlight, though, that R49 and R47 are jumpers, not resistors, as indicated by the zero ohm value noted on the schematic ("0R00"), and as stated explicitly in the parts list in the service manual. So analog signal ground and chassis/safety ground are connected directly together in this design, rather than through a low value resistor (which would lessen the chances of ground loop issues, as Atmasphere has stated in a number of past threads here).
Also, as indicated in the Rane paper you linked to earlier, ideally XLR pin 1 should be connected to chassis ground and NOT to signal ground, although in this case that is probably a moot point since both grounds are jumpered together.
So although the design is not ideal in a couple of respects, in ways that could conceivably contribute to hum problems depending in part on how grounds are handled in the component the amp is connected to, and perhaps also depending on whether XLR or RCA interconnections are used, my understanding (consistent with what is said in the Rane paper) is that lots of other designs are similarly flawed.
But we’ll soon see what the upshot is in this case.
Best regards,
-- Al