Living with unsolvable hum - Any audio detectives out there?

For over a year I have put up with a hum in my system, coming through the speakers (not chassis hum). I cannot make it go away. It seems to be related to the preamp because it stops when I replace the preamp. But I had my local tech hook up the preamp on his bench and it is quiet as a mouse. I've also corresponded with its designer, David Berning, who has been very responsive and helpful. But no luck solving it. I thought it may be related to the separate power supply's umbilical but David Berning said likely not. Earlier this year I even bought a star grounding component from Granite Audio and connected everything to it. Didn't work. After trying everything the engineer at Granite could think of (he was great), he was stumped too. These people have forgotten more than I'll ever know about the subject, so I gave up at that point and just lived with it. I had also tried everything they and a few knowledgeable friends have suggested (see below). But now I would like to take another swing at solving it. Any ideas? What kills me is that now I can't recall when it started, which would be very helpful to diagnose. The system sounds as good as I've ever had it now, and I LOVE the Berning preamp. So replacing it or other major components is not an attractive proposition for me.

For any intrepid detectives, here are the facts:

- Hum is typical 60 cycle sound- both channels equal volume of hum- loud enough to hear at the listening position, but just barely. Quite noticeable when standing at the rack.
- Hums with any source, not volume dependent, still hums with no source components attached (I even tried unplugged them from the wall too). But the hum stops if preamp is disconnected from amps.
- System plugs into a dedicated 20 amp line with eight plugs. Nothing else is on this circuit except my audio system. I had an electrician verify and tighten all the ground connections. The service is a relatively new 200 amp service. The electrician tested and found no ground issues or noise in the dedicated line.
- Tried shutting down all breakers in the house except my dedicated audio line. No effect, surprisingly. I had high hopes for that one!
- Tried cheater plug on everything including the preamp. No effect.
- Tried different interconnects between pre and power amps... No effect.
- Replaced all linestage tubes. No effect.
- Moved components around, moved the power supply, even used long interconnects to move the preamp three feet in front of the rack. No effect.
- Tried an extension cord to plug the preamp into a different AC circuit. No effect.
-The only thing I know of that could try, but have not tried, is replacing the power supply tubes, but I didn't bother because on the bench it made no noise for my tech.

My system:
- Power: Temporarily I'm using a Shunyata T6000 distributor (the hum existed prior to this, and the Shunyata didn't solve it). All Cardas Golden Ref or Golden power cords, except T6000 is plugged into the wall with Shunyata Sigma HC cord.
Analog: Koetsu Rosewood Signature Platinum, Jelco TK-850, Cardas Golden Cross phono cable
Digital: CEC transport and Audio Logic DAC, Golden Cross interconnect.
Preamp: Custom Berning Octal tube preamp with separate tube rectified switching power supply, built-in Jensen transformer MC stage at 24x gain (on the high side, I know, but it sounds amazing compared to other winding options)
-Power amps: Quicksilver v4 monos with KT150 tubes
-Two REL G2 subs (hum existed before them, and persists when they are disconnected and unplugged)
Somehow the interaction between the preamp and other components seems to be creating the problem. Source components don't seem to matter, but amps are Quicksilver v4 monos. Speakers are Verity Audio Parsifals. Interconnects, speaker cables and power cables are Cardas Golden Cross.
Speakers: Verity Audio Parsifal Encores. No surround sound or home theater.

OP before I read through this, did you pull the valves tighten the pin pockets and swap the rect valves? Did you pull the fuse and inspect the fuse pocket, clean it and replace the fuse. Did you replace the PC? Just asking?

Cable tv was the culprit in my system. Put a $15 isolator between cable and box, quiet as a mouse!!
I have not checked/cleaned the preamp fuse. Will do that first, as well as disconnecting wifi and other simple tests suggested, such as trying a backup amp to see if the interaction with my amps is a problem. Then check pre tube pins seatings. Then will see if I can take it to a friend's house. if it hums at friend's, maybe replace power supply tubes. Then will embark on other ideas mentioned.

Can't thank all you guys enough for your time and ideas!
A common wiring arrangement in pro audio to stop ground loops is to lift the ground of interconnect cables at one end. We did it all the time building amp racks and outbound racks. We would lift the ground on the inputs to the power amps when linking them together and from the outputs of the crossover/processer. Easy to do with balanced XLR connectors. We always did it on the input end of the cable. We also never lifted power grounds, it was forbiden by my boss at the time.  I have never done it with unbalanced RCA cables but it is worth a try.

Lifting the signal ground is a fine idea, when there are multiple signal wires.

These are mono amps, so I would suggest the OP try cheater plugs on one or the other amp first, and if that works, get a hum buster (or whatever they are called).
Actually, the OP should unplug 1 mono amp from the power completely, and see if this doesn't fix it.