Hum in ARC Ref 3 not on 0 but 1-103 light buzz

Well, if the cheater plug failed to relieve the hum, then IMO it is not a ground loop hum

Have you tried listening with the LED turned off? There is noise given off by the LED. Seems odd to me that a ground loop hum starts at 250 hours of tube life. Have you called Leonard at ARC? If the cheater plug makes it go away, then yes it is a ground loop hum BUT I would never recommend using a cheater plug for fear of electrocution or frying your Ref 3.
I would definitely check your tubes and I bet ARC would send you replacements with only 250 hours of use ( I love that tube timer on the Ref3)

There is no way that a change in cable will make that hum disappear. If you believe that, then I have a bridge to sell you here in San Francisco

Broederen

A simple solution that I use in my system when I added the pair of Gotham subs and was thus driving 2 loads in parallel via the Ref 3---thus creating some strain on the Ref 3 was to add the Tube Buffer and its separate Power Supply from Musical fidelity. This is a very simple, effective and inexpensive fix. Musical Fidelity does not make the Tube Buffer any more but if you are interested send me an e-mail and I will tell you of a dealer who has them new and in the box.
The Tube Buffer has an input impedance of 470 KOhms and will solve your problem. If you read the recent posts in my system you will find a description as to how this can be done

Broederen

I forwarded the name of the dealer to you by way of e-mail