persistent 60 cycle hum only on tube gear

@OP As above, you don't want your amplifiers grounded separately to the rest of the system. Apart from that, does your ARC pre mum when connected to the Bryston power amps. If not, there is always the possibility of bad filter caps in the ARC power amps. But ground loops still remain the main suspect.

The power supply caps on either or both of your ARC gear may be dying. Since everything thing else is working fine that is what I would check.

@yoyoyaya : I posted my reply before I saw your post! Serendipity! Power supply caps are the Achilles Heel of audio gear. It is apparent when they start to die.

+1 jasonbourne71. @OP Power amp caps are more likely to fail than pre's - though with high temps in tubed gear, you never know. But if there is no hum with the pre connected to the brystons, then the finger points towards the ARC power amps - or a ground loop. 

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.

@allenf1963 

Thank you so much for the kind words, I can’t try to be smart but I can try to be kind. 

Can you explain to me why an isolated ground is prohibited by the electrical code?

Let me see if I can do a better job than AI here. :) 

It boils down to "what is the ground conductor in a receptacle meant to do?"

A ground pin on a receptacle should provide a high quality connection back to the service panel where the ground is bonded to the house neutral.  This means low resistance.  The idea is that should a fault occur from the electricity flowing through your appliance to the outer chassis the ground should whisk that voltage away and cause a breaker trip.  

Audiophiles get very tunnel visioned when it comes to ground wires, never before thinking about them until they start chasing some real or imagined problem with their gear.  To these audiophiles it’s like "Aha!! Thomas Edison himself along with the American industro-capitalist cabal are conspiring to keep me from my ideal sound!!"  And having realized this undeniable truth then set about to undermine it at every opportunity! 

So, back to reality and why it’s bad/not up to code.  The quick answer is that an isolated ground conductor wont’ function as intended.  It won’t trip the breaker during a fault and tripping the breaker and keeping the voltage off the chassis is exactly what it should do.

Without getting into math, just consider that the dirt around your home is a poor conductor, or at least, incredibly variable quality of conductor. If it were a good conductor we’d probably have speaker cables made of dirt, but I digress.   Point is, you can’t use the dirt in your home as a guaranteed high quality, low resistance path back to your service entrance ground point.

Imagine you have a washing machine, with a fault.  Meaning, the hot wire (black or red) is conducting to the outer metal case, the chassis, of the washer.  The quality of this bad situation varies a lot, sometimes it’s very slight and sometimes it’s lethal.  If it's really bad we call it a "hard" fault. 

If that washer's chassis is attached to an excellent conductor back to the ground at the service entrance then 100% of that current is conducted through it, and the voltage at the outer chassis is 0.  Meaning when you touch it you touch the voltage at the ground.  

If the fault is "hard" it will also trip the breaker, exactly as intended.

Now lets imagine a washing machine on an "isolated ground."  As you move that new ground rod physically away from the house ground rod the voltage on the chassis rises, and the likelihood of the breaker tripping before you are shocked to death decreases.   Something else to keep in mind, is that a typical home circuit is 15 Amps, but 0.008 Amps in the right circumstances can be lethal. 

Also, 15 Amps through a delicate RCA connector can start a fire when it melts explosively.  This can happen if the RCA ground becomes energized and the appliance it is connected to is not properly grounded.  I’ve seen very similar situations described here on Audiogon.  

To summarize, I’m not sure if my explanation is clear enough, but the truth is that it’s forbidden by the National Electric Code and for exceedingly good reasons.  I hope my explanation helps you understand the whys of that.  I 100% agree with the NEC and encourage the OP and anyone else to high a qualified electrician when in doubt.