Hi kazkru,
Very good trouble-shooting on your part. Now you can try connecting a separate ground wire to a chassis screw on your tube receiver amp. Run the ground wire to a known ground if you have one (a cold water pipe?) and see if the hum disappears when you touch the ground wire to the ground, or you could try the center screw on a household AC outlet, if it's properly grounded.
An alternative would be to run the ground wire to a chassis screw on the rear panel of the subwoofer amp. That may eliminate the hum. I once had to run a separate ground wire from a big old Macintosh tube power amp chassis to the pipe underneath a basement bar installation. The reviewer/journalist (an icon on Canada's CBC radio network; for those with long memories, it was Clyde Gilmour of the network radio show, "Gilmour's Albums") was so grateful to me, he mentioned me on-air and gave the magazine which I edited at the time (Sound Canada) a nice plug.
A bit of a digression, but I mention it to illustrate the extreme measures that are sometimes necessary to get rid of hum.
Regards,
Alan
