Hello Harv-e,
I got into your repair yesterday afternoon and this morning, and I found that a couple of resistors may have been damaged on the 12BH7 socket, so I replaced them with more robust parts.
I also found some other parts that may have been a bit loose, and I tightened them all down to prevent any further contact issues.
Also, the ground wire from one of the input RCA jacks to the terminal strip was loose in the terminal strip and not soldered down. This presented us with significant hum while running the amp in until I was able to find it by poking around with a pencil. This is most likely to be the primary issue you were facing, and when you ended up replacing the tubes, the physical jarring of the Eros would have temporarily re-established this connection for you for a while.
I did end up replacing one of the C4S boards in the process of debugging, but in retrospect it wasn't at all necessary.
We'll keep running the amp for a few days with the inputs shorted and the outputs driving speakers so we can be sure that you won't have any additional noise issues.
Sincerely,
PB