persistent 60 cycle hum only on tube gear

I am confused, did the problem exist before you established a second ground and/or clipped the ground wire to your breaker box? BTW, pretty sure separate grounds are frowned upon, and can pose some risks.

Using an isolated ground is expressly prohibited by the National Electrical Code.  It is unsafe and may also prevent any surge protection from working correctly.  For Audio, it is also completely unnecessary.   You would have seriously created a less dangerous situation by just lifting the ground altogether, which is also a bad idea.

It’s highly likely that you have an issue with the voltage differential between your new ground and neutral, and some gear may be more sensitive to others, causing the hum. 

There is exactly 1 condition under which you can use an isolated ground, and that is when you have an isolated power generator, such as solar, or battery back up that is not otherwise bonded to the incoming grounding electrode system.  So if your only priority is to have an isolated ground, then you have to have an isolated power generation system as well.  

Of course, should something happen to you, or you sell the house, this wiring mistake will live on with no one aware.  

Erik Squires pointed to one issue I was thinking. Another that took me a while to find was an internet router/wifi too close to the tube preamp.

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.