persistent 60 cycle hum only on tube gear

I’m not saying an auxiliary grounding electrode is an effective ground-fault current path — I’m saying it can become an unintended parallel current path during abnormal conditions, which is why assuming small-gauge wiring is adequate is risky. Of all the places where audiophiles should overbuild, grounding is one of them.

I suggest that the argument on Audiogon is moot, what will matter is what the inspector thinks, and good luck getting them to agree it's "auxuiliary" instead of "supplemental."  

If you can replace the electrolytic filter capacitors in the power supplies in the vacuum tube amp and preamp with metalized polypropylene capacitors, they will last forever because they do not go bad even when not used every few years the way the inferior electrolytic capacitors do, they have better ESR, and they self-heal if a leak should happen.

I suffered a similar hum as the OP with my Air Tight gear. As an electrical engineer, am somewhat conversant with the problems creating the hum and nothing worked. With the help of the very responsive folks at Shunyata, we finally came to the conclusion that the tubes were picking up RF broadcast by the Verizon Fios router. I tried moving it to different locations, but short of actually building a "screen room" around the tube gear, nothing worked. At Shunyata's suggestion, I swapped to solid-state and, zero noise. Been a tube-roller for 20yr, but am just as happy with solid-state.

First: there is a difference between hum (60Hz) and buzz (120Hz). Bad PS caps result in buzz. I have built many tube amps and none of them produced any hum. Hum can result from magnetic coupling between PS transformers ( chokes) and audio transformers. Also, poor routing of AC mains and filament lines in the amp. In rare cases heater/cathode leakage in tube with AC filament supply (this can be minimized by balanced, grounded heater supply and also elevating the voltage of the heater supply above ground). I would assume that the engineers at ARC dealt with these issues.

Then there are ground loops. These are caused when the grounded chassis of two units are at slightly different potentials. This results in a ground current that causes hum. The cure is to isolate the circuit ground from the earth ground. Some use back to back diodes; I use a CL-60 thermistor in parallel with a 1uF film cap. If you are using balanced cables this connection includes a ground connection. Depending on how the balanced outputs and inputs are implemented and whether the circuit grounds of the two units are isolated from earth ground this could be an issue.

Side note: my line and phono stages have floating transformer outputs and I have no issues with ground loops. I'm sure Ralph will chime in here with "transformers are bad". Cheap ones are; good ones are expensive.