Loud Audible Hum/Noise from C/J Tube Amps

it's pretty common with many amps. I generally ground my preamp only, and lift the ground on power amps, assuring they have an appropriate ground connection through the IC to the preamp.

This can have something to do with the wiring/grounding of your place. I used an MV55 and had the same problem, fixed by the same cure, when I lived in one place, but at the next place no problem even when the amp was grounded. (In both places the preamp and power amp were each plugged into the same central power distributor.) The only thing that puzzles me about your experience is that you say you don't hear the hum until you turn on the preamp. With mine the power amp hum was heard whether or not the preamp was turned on.

cable tv sometimes causes this. try taking the coax cable out of the cable box then turn the system on without the cheater plugs and see if the problem is still there.