Two Turntables and Hum Problems - Help Please

Three suggestions:
First hum...the ground wire may not be the cause of your hum it could be the interconnect between the pre amp and turntable. This can occur if the interconnect has been placed too close to a power cord of any kind.

Note placing the power cord or the interconnects at right angles to each other may not solve your hum problem. Only distance between the power cord and interconnect will make a difference.

Second...
Replace the 30 foot section of ground wire with a single contiguous piece of 20 to 24 gauge wire.

Third...
When your cartriage output is only 3 to 4 mv, 30 feet is a long way for the signal to travel without picking up noise. Can you use a step up transformer for the cartriage to boost the signal over this distance?

I would also make sure that your interconnects are shielded from the TT to phono pre.

the subs could be a problem too, as the OP offered up.

In addition to the good comments above, another reason that a 30 foot run from turntable to phono stage is unreasonable is cable capacitance. With a moving magnet or other high output cartridge, sonics will be significantly degraded unless you use a cable having extremely low capacitance per unit length AND the cartridge is specified to work into high load capacitances. With a low output moving coil cartridge that may or may not be an issue, but the likelihood of noise problems will increase significantly.

Get a separate phono stage, as Stan and Elizabeth suggested.

Regards,
-- Al