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

Should be close as possible. Above comments correct, use a good phono cable to connect to pre...hum can be reduced greatly by turning the sut....if you cannot hear hum at listening position, then there is No problem.

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.