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.