60 Cycle Hum in Tube Amp

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Does the level of the hum change when you adjust the volume?  I'm guessing if you can gain stage around it then it does, which suggests that it's introduced at an early stage and could be a bad preamp tube.

Where are you hearing your interference with the mic - through monitors? Are you using an external audio interface? An old pair of monitors I had used to pick up hard drive interference on occasion, it was only in the monitoring chain and not in the actual recording. That might be something to check. I cured that problem with an isolating transformer box. I'm assuming you're definitely using balanced mic cables?

Cheers

It sounds like one of the preamp tubes might be the culprit. A pair of 12AX7s I think? You might try swapping them out first. If it solves it then you're all good, if not then you can look a little deeper. You could buy one (or two - doesn' hurt to have spares) and try it in each position, to see if one or both are at fault.

Ok so it's in your recording chain, it could be a bad mic cable - good first place to look.  Try another just to eliminate it. It could be a ground loop or even bad grounding in your home that could be the problem. Have you tried recording with your Mac in a different location? Seems strange the SM7B would be quieter, but then it does need a hefty amount of gain in the first place

Cheers

There is a 60 & 120 cycle component in all amps to some degree at a idle level.  You might try an attenuated output between amp and speaker to lower this level.  Also lowering eq tone stack settings also reduce  drive to the power amp. Causing more idle amp noise presence.  Another solution is always denoise plugin. 

It sounds like you might be using single coil pickups rather than hum bucking.  Computer noise is also getting into guitar pickup and through power supplies.  Balance power can help.