persistent 60 cycle hum only on tube gear

Try unplugging every SMPS power supply in your house (wall warts or bricks, i.e., laptop chargers, desk top speakers, etc.).

Does your ARC gear have a ground plug on the power entry? If you have a DMM. measure pin 1 of each XLR connector to the power inlet ground to check for chassis ground. It should read continuity or a low value R such as 10 ohms. If both amp and preamp XLR jacks are chassis grounded and both have a ground plug, put a cheater plug on the amp. If still humming put the cheater plug on the preamp. If either works in solving the hum, then you most likely have a wiring issue in one of the units that compromises the grounding path.

I had hum like that that I could not figure out until I stumbled on the fact that the mesh network node was too close. Once moved to the other side of the room, the hum went away. In my case too close meant three or 4 feet away.

This is not helpful except in I feel your pain. I had that same problem with a pair of ARC amps years ago. I was not able to fix it no matter what I tried, so I sold them and stayed away from tubes for a long time. I have always been confounded by the hum of those amps. I hope you are able to fix the problem.

John 

@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.