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

Cable from SUT to phono stage should be kept short (less than 1 ft) to avoid capacitive loading/hi frequency roll off. Ground wire from SUT to phono.

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?