I have a $100 turntable that works perfectly on customers system. The $300 turntable he purchased online has a pronounced hum that varies with the volume. Naturally I am still somehow on the hook for making his turntable work with the system... bc he likes the turntable.

At this time could someone please take this empty bottle of whiskey and smash it over my head?

Both turntables are made without ground wires or any place to attach a ground wire. There's also a built-in preamp and a switch to engage/disengage it.
We are talking about a 70 foot run to the preamp.

I ALSO RAN BALANCED AUDIO and used a high quality active balancer before the run and used balanced inputs on the preamp that didn't help.

Again... my turntable works perfectly, no hum. His turntable hums no matter what I do, even if I hook it directly to the preamp with short cables, even if I use the phono level out of turntable and go into good preamp (tried both balanced and unbalanced outputs out of preamp)