persistent 60 cycle hum only on tube gear

@erik_squires 

Erik, I always enjoy reading your posts and replies.  You are very astute and intelligent, and your posts don't look down on others who may not be as up to date on the subject at hand.  Can you explain to me why an isolated ground is prohibited by the electrical code?  I am a babe in the dark when it comes to most things dealing with electricity -- I avoid that issue like the plague (I got a pretty nasty shock when I was a kid by a wire that wasn't grounded properly, so I'm pretty phobic about it).

I'll shut up and listen!  Thanks.

iFi makes a ground hum gadget. Sorry I don’t have model info, but it eliminated hum in my complex system.

I wondered and googled

"why is an isolated ground is prohibited by the electrical code?"  The result is to long to paste here. 

There is a rather exhaustive explanation of why it might be a bad idea.

Remove the groundmaster/separate ground, and try different xlr's.. I guess ground loop or bad cable/s.

If all else fails, I’d check the transformers.  Some are just noisy but it is inaudible when playing. My tube gear uses all Hashimoto transfers and we needed ‘pots” to drop the hum.