persistent 60 cycle hum only on tube gear

First: there is a difference between hum (60Hz) and buzz (120Hz). Bad PS caps result in buzz. I have built many tube amps and none of them produced any hum. Hum can result from magnetic coupling between PS transformers ( chokes) and audio transformers. Also, poor routing of AC mains and filament lines in the amp. In rare cases heater/cathode leakage in tube with AC filament supply (this can be minimized by balanced, grounded heater supply and also elevating the voltage of the heater supply above ground). I would assume that the engineers at ARC dealt with these issues.

Then there are ground loops. These are caused when the grounded chassis of two units are at slightly different potentials. This results in a ground current that causes hum. The cure is to isolate the circuit ground from the earth ground. Some use back to back diodes; I use a CL-60 thermistor in parallel with a 1uF film cap. If you are using balanced cables this connection includes a ground connection. Depending on how the balanced outputs and inputs are implemented and whether the circuit grounds of the two units are isolated from earth ground this could be an issue.

Side note: my line and phono stages have floating transformer outputs and I have no issues with ground loops. I'm sure Ralph will chime in here with "transformers are bad". Cheap ones are; good ones are expensive.

This “discussion” is a fine example of Agone fora finery! Pro bono unius, pro bono omnium! Rock on!

@yesiam_a_pirate (Pirate, eh?!!) Looking at the photo of your equipment you show the AMP and other gear standing vertically. Could this contribute? The kids in the @erik_squires and company genius section ( I never got past PV=nRT) are far above my turn it on, plop down and groove out classification. But if your equipment is aged, well then. Also if wifi close by it needs to go by by. 

Apropos of nothing that’s very cool flooring in yr. room.  

I actually had this problem as well, but only with certain tubes (a solid brand of new tubes, not NOS).  Essentially, though the tubes were a matched pair, I was only getting hiss from the left channel.  When I switched the location of that tube with its matched sibling, the sound switched to the right channel.  I actually went through another set of matched tubes from the same company, and had the same issue...albeit, it was slightly more quiet than the first time.

After some conversations with the tech at the company, we concluded it was mostly likely due to the high sensitivity of my speakers (Klipsch Heresy IVs) He acknowledged tubes do have a slightly higher noise floor than solid state.

As a result, he screened through and found a new matched pair that had substantially less noise than their standard lot.

Once I put them into my tube preamp, there was nothing but bliss.

AC heaters in tube amps will naturally hum to one degree or another. 

 Small signal tube caused 60Hz hum.

https://www.youtube.com/watch?v=GrVtX0QGNls