Phono Hum Question


I have a very low grade hum that disappears when the phono lead ground wire touches the ground sleeve of either phone rca cables. Completely gone. It’s a new dedicated room, outlets have isolated grounds, one 20 amp circuit. The curcuit goes through a outdoor generator transfer switch. I know, I read Mikeys article. System is otherwise silent. Cartridge AT-Art-9, phono leads Audio Sensibility Statement Silver, Gold note PSU-10, PH-10. Balanced to LTA Integrated. In fact, when the Gold notes are grounded together with or without the phono lead ground, hum worsens to not tolerable. The rebel in me says solder the phono lead ground to the negative outer sleeve but I probably won’t. Interestingly enough, when the phono stage is plugged into a separate circuit, no hum at all, phono ground changes nothing. Where might the ground potential be different? Thank you all. 
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I was assuming the ground wire supplied with the tonearm cables grounded the tonearm. Looking under the tonearm mounting plate I see the ground wire for the plate in which the tonearm is not isolated. It will take me a day or two to make sure that lead is grounded properly. That just might be it. Thank you both. 
The ground wire is the tonearm tube amongst other things. If not grounded it will induce noise into the cartridge signal for which it is supposed to be the shield. Just ground the wire- that's what its for.
OP: 
Where might the ground potential be different? 


Also OP, same post:
 when the phono stage is plugged into a separate circuit, no hum at all, phono ground changes nothing.

States the answer, then asks the question he just answered. Fascinating. 

The way it was actually written in the original:
 when the phono stage is plugged into a separate circuit, no hum at all, phono ground changes nothing. Where might the ground potential be different? 

While we can only guess all the places it MIGHT be different, you just told us where it IS different. Like I 
said, fascinating.
I just sold an old AQ din cable that had 2 ground wires. One from the cable shields (outer) and one from chassis. They were connected together by someone (?)
Voila
You may think you painted a clear picture of your ground scheme, but I’m a bit confused. The only thing I do understand is that the source of AC makes a key difference. I would start with that observation and work backwards.
You know how to get rid of the hum and enjoy silent bliss. So what is the problem?