Phono cable not caps to le with preamp?

Definitely.

Yes, ground. Need to run a ground from the turntable to the phonostage. Long ago I had a turntable without a grounding wire. I found a nut connected to the chassis on the bottom of the turntable and ran to the ground connector on the phono stage. I never heard it again.

@ghdprentice  I have done this yes. I also tried various different cables and plugged into different outlets. I also moved the power supply as far away as the umbilical allows and moved the unit itself away from everything else.

my cables are shielded and have never had this issue.

if I unplug the input RCA’s it’s dead silent. If I plug them in it’s an overdriven buzzing sound that is affected by the volume on the line stage. 

I just had a thought. Since the phono and power supply are in wood boxes instead of metal would they’ve more susceptible to interference?

A VERY loud hum is possibly due to a more major ground fault than the hum that typically results from failure to ground the TT or tonearm, in my experience. If the usual suspects don't work at all, I would look at either the cables (maybe the solder joint between the cable itself and the grounded barrel on one of the RCA connectors came loose) or the new phono stage itself.  Crack open the chassis and have a look at the solder joints to the outputs.  But one huge question: Is the hum in both channels or in one channel?  If in both channels, then my hypothesis loses some credibility.