This might be slightly off-topic here, but since we're talking subs, this might helpful to some who have trouble with sub hum.
I was able to largely quell my 2 X Velo subs' hum (I suspect was from my cable TV feed) by grounding the outside barrel of one of each sub's unused RCA jacks to the 3rd prong of the house power supply. It worked only if the plug in the wall socket was positioned 'just right'. If it was disturbed at all, the hum returned.
So, in frustration I tried changing that connection to the outside barrel of an unused RCA Input on my Sony XBR TV - voila, hum completely gone.
Thankfully, no more wiggling the plug in the wall socket...