VAC Phi 300.1a Single Channel Ground Loop Hum

I borrowed a Vac Phi 300.1a amp to test in my system.  What a beast to move around but that's another story...

With the amp in my system instead of PS Audio BHK 250 I get ground loop hum in a single channel (right).  I've swapped input tubes, no change.  Speaker cables not crossing power cords.  Same hum whether the preamp (NAT Audio Magnetic) is connected to the amp or not so I know it's not the preamp.  VAC passes audio in right channel but the hum is pretty loud.  The amp was a trade-in to the dealer who didn't bench test it before letting me borrow it.  I guess could be a bad outuput tube but it will be kind of a PITA to swap them around because it's on a shelf in my rack.

I've never experienced single channel ground loop noise before but my dealer says he has with a couple of tube amps.  I've got no financial exposure since I'm borrowing it but I really did want to hear it especially since it was such a bear to install it (~130lbs of amplifier).  Any ideas?

Thanks much!

Ian

P.S. Not that it means anything but Kevin Hayes from VAC was a classmate of mine in grad school.  I was his TA in a quant course.  I had no idea then that he was into audio.  Wish I had.  When I met him at a show about 10 years after we graduated he was very cool.