That's a good point which I forgot to mention. Nearby mains transformers can induce hum into the SUT, though moving the SUT only a few inches (any maybe turning it through a few degrees) can reduce the hum dramatically.
Over the years I've had lots of people contact me regarding step-up transformers which hum, and they nearly always start by saying they have "an earth loop" causing hum. Sometimes that's the case but often it isn't. The other two most common causes of hum are:
1) placing the SUT too close to a mains transformer, and
2) using unshielded interconnects between the SUT and phonostage.