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

The short answer is no, you need not tolerate hum just because your SUT is close to your phono stage. In fact ideally you want the two to be as close together as practical.

External SUT should be hum free -

Use low capacitance shielded cable between SUT/phono

Make sure SUT chassis is earthed to phono via additional earth wire

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.