How far is your SUT from your Phono Pre/Preamp?

Got it. I'll try other cables today to see if that's what's causing the hum.

Dear @dhcod  : The orientation/position of the cables ( an even of the SUT it self. You can rotate it for example.  ) running is something to try. Sometimes running the cables in a " weird " position makes the hum disappears. You can try with the cables you are using and that " makes " hum.

Regards and enjoy the MUSIC NOT DISTORTIONS,

R.

All good advice here.

In addition to Raul's comments you might also try a different orientation of the SUT.

You don’t happen to have the SUT plugged into a different electrical outlet? 

I've spent the morning doing a bunch of things, reorienting, finding weird angles, and changing cables to shielded cables. Cables make no difference at all. There's definitely one spot that I found to place the SUT that is the quietest and it's about 14 inches away which is good... but there's still a noticeable hum, although only within 3 feet of the speakers, not in the listening position.

My question now is whether the hum is the fault of the SUT or the phono preamp? Will any SUT that gets near my phono preamp cause a problem? Probably need another SUT to figure that out.