I have the dreaded hum

I have a humming that occurs on one channel only. It happens when TT is off or on. When you swap TT leads it stays on same channel. When you swap interconnects from pre-amp to amp it switches channels. Only occurs when pre-amp is on phono input and set to moving coil. If you disconnect TT leads it stops. Equipment and set-up is all new. I've moved the equip around, no change.
I sent the pre-amp back to the dealer a while back and he said it was ok.
Today I bought a diff phono stage and the hum went away.
Dealer still thinks the pre-amp is ok.
What else could it be?
Tube amp and pre-amp