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?

A couple of questions, first, did this just start or have you had the problem as long as you have had the amps? If it just started, did any system parameters change?

When you say that the hum is present with the amp not connected to the preamp, are you shorting the input to the amp or just leaving it with nothing plugged into it?

What is the difference between a 60HZ hum and a “pseudo 60HZ” hum?

My sense is that it is one, or more, of the power supply caps in the offending amp. Often there is no visual indication that the caps have gone wrong, but sometimes they bulge, or leak. Have you inspected them for irregularities? If you can measure the leakage in the caps that would be useful, otherwise my sense is that a trip to the stereo doctor is in order. I would look at the cathode bypass caps while I was at it.
If nothing is hooked up to the amp this issue points to the amp itself.

What happens when you short the input of the amp? If it goes quiet, then the real issue is whether it has a hum when you connect the preamp to it.

If it does at that point, then it needs to be serviced. If there is no hum or so slight that you have to be within 6 inches of the speaker to hear it, I wouldn't worry about it.

Found it!  Freaking cable box sitting one shelf under the noisy amp.  Must be broadcasting RF.  Moved and solved.  I was just before taking the back off the amp.
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