Ask your question over at the VPI forum, maybe Harry or Mat will chime in.
I would try using a "cheater" adapter (easily bought at Home Depot for less than a dollar, which would allow you to turn the plug 180 degrees and/or even eliminating the ground lug. You will find the offending circuit....just patiently experiment. I don't know the Phoenix stuff intimately....make sure you can eliminate the ground, or plug reversal without damage. When you find the problem, if a ground has to be eliminated, I would remove the wall plug, and disconnect the ground wire...those cheaters can make the sound grainy, but you don't care...you're just trying to find the ground loop. I would mark the wall outlet with a small piece of paper saying the ground was altered.
Trytone.....I suggested the cheater to find where the ground loupe is. Try it on the turntable first (I doubt it will help there)..... also try it on your other black boxes until you find the culprit. In my system (Ayre amp, preamp, Universal silver disc player, Vandersteen speakers (which plug into the wall), and dedicated headphone amp)...if the headphone amp is not plugged in, I get a hum (with the 2nd preamp output connected).... You just have to wade through the system ....the joys of audiophiledom. Plug the 3 way plug you are testing into the "cheater" which allows you to reverse the plug (turn it 180 degrees) and/or eliminate the 3rd ground wire (the rounded metal spade on the plug...as opposed to the 2 flat ones (one wider than the other).
I had a Classic fairly soon after they came out and it had a turn on thump that which I called Harry W. about. He had me switch a capacitor to one with a different value and the problem was solved. Call Harry. He is very good at diagnosing the problem and more importantly, at providing the necessary fix.
I too had a similar hum problem when I switched out my Rega RP6 with my new VPI Scout ( 2016 ), using the Phoenix PSU Roadrunner Tachometer. After trying to isolate the hum problem and various scenarios for eliminating the ground loop I had no success. I did find the culprit however, it was the wall wart power supply to the PSU. I removed it from the system and the hum went away-dead quiet. I made the difficult decision of selling it, I opted for the quiet! The last readings I was getting from the PSU / Tachometer set up was a pretty steady 33.333. So I'm assuming the Scout is spot on. I hear no pitch derivations and every recording I play sounds just like I always heard it. I hope this helps. I'm not suggesting you do the same but I can say that I just enjoy the music now and I'm not obsessed with micro speed variances that I will never hear.