Two Turntables and Hum Problems - Help Please

Let me understand, you are running 30' of ICs BEFORE the signal gets to the phono stage? Remember the cartridge generates a VERY low level signal compared to a CD player, tuner, etc. If I had to place my tables that far away I would buy a cheap phono stage to put next to them; this very long cable run may be the source of your hum; in any case it is a big no-no.

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.