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.

The issue is the ferrite toroidals are not very effective at 60hz. More likely they take out the effects of 120Hz events, like dimmer turn on but do little about 60Hz fundamentals. Hence a small isolation transformer on the preamp as a better test.  Attenuation at 60hz with that ferrite toroid will be near 0.
2 items I haven't seen mentioned:
1-Did you move your system to a new room?  I know it's not easy to say the least.  You could run a really long extension cord from another side of the house?  Not sure it will help.
2-Did you try different amps.  It might just be a compatibility issue.  Don't use that junk receiver but borrow a nice high output power amp.
Your system has great components but that doesn't mean the kids will all get along well.
Wow Evil Stu thanks for all that! I don't have much slack in the power cord and it is heavy-ish Cardas Golden Ref. Hmm.
Good Stu
Couple more thoughts and experiments:

* I did a brief read on Shunyata and their power distributors. I might have missed it, but they did not call out specifically how their internal distributors actually work and corresponding components. It might be worth doing a YouTube search to see if someone has opened up a T6000 to actually see what components Shunyata is using in your distributor. The website refers to toroids, transformers and capacitors, but it is really unclear what actual parts are used in this device. If no one has opened up their the first to do it and see if they have transformers, caps, toroids or if this is merely a "power strip"

* Cheap and easy....borrow an AC line conditioner, regenerator, etc. from a friend to see if it makes a difference, for your preamp. As usual, scientific method (plug this borrowed line conditioner into same outlet that your preamp is currently using and only plug the preamp into this device. If no difference, then one by one add more of your stereo components. You are trying to see if another device (close proximity) is creating the noise.

* I found, multiple number of man made /house appliance and device RF noise sources could exist and no silver bullet. As audio2designs reminded me, even devices such as dimmer switches can be a source of noise transmission that is picked up and amplified by your preamp.

* The torroid I specifically used (and was recommended), mix 31, is designed to trap man made noise low frequency such as transformers, walwarts, etc. If you do a quick google search to see what other people have done and their results, you will see pretty good results. Not a 100%, but again like insurance, you have to have layers of filtering for pesky noise inducing situations. That is why I could easily hear reduction in noise in my music room. Will I have to explore other possible capacitors, transformers, toroid mixes? Yes. Will only one component solve all MY issues....nope. Hence, keep an open mind to the different possible noise trap solutions and the different sources of noises...which leads me to....

* A cheap and easy way to "hunt" for man made / house noise is using an old battery operated pocket AM radio. Go outside away from your home and away from household noise inducing devices. Set the AM radio to the top of the band, 1600 (1600 kilohertz), or something close to that range that does not have a broadcast signal. Listen and learn for the speaker noise and pitch on that non broadcast signal. Now go back into your stereo room and put the radio near each transformer, device, light switch, light, computer, etc.. Your going to use this device as a noise transmission detector.

Slowly turn the radio 90 degrees relative to the device and within 6 inches from each household device, switch, power strip, walwart, etc. If you hear a change in pitch/noise.....your on to something. If it is a light dimmer, for example, maybe it is an old dimmer that needs to be replaced. If it is a new device, then you will need to get creative on how to eliminate the noise transmission from that device. In my garage I have a battery tender for my car. The battery tender is relatively new but it is located near my ham gear. I A/B tested (before toroid / after toroid) with the toroid and noticed a small but perceivable reduction in noise. Again, you may have several different devices that contribute to the noise so you have to prepared to hunt for different noise sources and use different techniques to suppress / kill noise.

Happy hunting, Mr. Wick.

Not evil...I am just curious.  ;)