Hum Super Sleuth Needed

OK guys, not my first rodeo but this one has me scratching my head.  I have a pair of Coincident Frankenstein-MkII monoblocks.  One is dead quiet, one has a psudo 60Hz hum with the amp hooked to the speaker and no input hooked up.  Here is what I have tried thus far:

- Cheater plug on power cord to lift ground (no change)  If this had worked I would just get a Hum-X and be done.

- Swapped all three tubes between amps (no change)

So if it's not a ground loop on incoming power and not a ground loop on the RCA input (unhooked) what can it be?  Possibly a bad filter inside the amp?  Anyone have ideas?