I have a hum using the phono input only when I place the arm on a discdoes this mean you don't have the hum when the cartridge/tonearm are resting on the tonearm holder, but the power is turned on? If you can power the system on and there's no hum except when the cartridge is placed on the disc, I don't think this is a grounding problem. It may be a problem of feedback from the motor through the cartridge - which is the same problem I have with a Grado cartridge on my Heybrook table. It increases in volume the closer the cartridge gets to the center of the platter (and the motor). Does this sound familiar?
maybe it's also possible that there's a short in your tonearm wiring that results from binding as the arm is moved??