Do you have access to a UPS generating its current from a battery? That could isolate completely the sub from your electrical system if that proves to be the problem.
I know a few people in the movie industry who are using that type of solution to fix strange electrical problems on location.
Many UPSes use "stepped sine" output, which can also cause transformer hum. It depends on how susceptible the transformer is, and how bad the power is in the house. Sometimes there is so much noise on the 60 Hz sine wave of the house, that a stepped sine from a UPS is actually cleaner.
