M, since breaking the loop at the sub power cord works, but you don't want to risk the sub amp possibly developing a short and not being grounded except through the sub cable, try breaking it at the sub cable. Pull the sub cable out of the input far enough so that the center conductor still makes contact but the outer grounding shield doesn't; see if this stops the hum. If it does, try to fashion a little strip of plastic or even paper to keep the grounding shield from making contact even when it's fully inserted. If it works, cheap. If not, there have to be Canadian sources which sell isolators.
