Apologies, I'm finally coming back to troubleshoot this. Using my MacBook, I've played a 60 Hz tone, full volume with the balance and volume pots in the middle and am only reading 50 mV on both top and bottom middle lugs of the volume pot. Between ground and the outer upper and lower lugs on the volume pot both read 250 mV.
I did notice this evening, the temperature of the 2A3 tube on the faulty size is running ~100 degrees F cooler than the functioning right side, if this would help isolate the issue.