Tubes trouble?

Dill’s idea is not a bad one. A shield should both quell microphonics and shield from RF. (But the tubes in an ARC may already be shielded.) You might also check that the unit is well grounded. When you touch it, your body affords another path to ground. Sounds like microphonics though. All tubes are to some degree microphonic. You might try moving the tubes that are nearest to the phono inputs to the more distant position from the phono input and move those distant tubes close to the photo input; in other words swap them in pairs. This is assuming they’re all of one type, e.g., all 6922s or whatever.

I doubt that this is a tube problem. My first guess would be that it is somewhere in the grounding or wires (IC's). Check to make sure all of your IC's are not near any transformers, etc and all connections are solid, especially grounds from your TT and phono stage.

I agree with @newbee it doesn’t sound like a tube problem, good advice on likely fixes.

But for all my ARC equipment I have an extra set of tubes. If a funny noise comes up in a single channel, I change the tubes in that channel (or both if I don’t know which is which. If in both channels then the 6550. If this doesn’t work I change all the tubes just to be absolutely sure it is not a tube problem. If at any time the problem goes away, I swap in the old until I find the little bugger.

If all tubes are fine it is something else. For me it never has been.

To me this sounds like a mechanical hum. First thing to check would be the screws that hold the circuit boards and transformer are snugged. Then I would call AR tech support and ask if it’s okay to remove one or all of the tubes and then power it on. This way you could remove one tube at a time and see if the hum goes away -- and you know it’s the tubes. (I should mention power off and unplug the unit each time before you remove any tube).

Not a good idea to remove tubes leaving a socket or sockets empty, and then power on. Unless of course ARC directly gives their verbal consent.  Also, even if you could get away with it without damaging the power supply or downstream equipment, it would be difficult to assign blame for the hum, because you've removed the pathway (to the speakers) by which you detected the hum in the first place. So you'd likely get a false positive result.

Something about the way the OP described the "humming" makes me think it is not a 60Hz or 120Hz noise but instead a mix of frequencies, which you can get with a microphonic tube or a tube picking up RF.  Also, those 6922s can do that.