Mark, yes there's a recommendation: since you've reported no hum at a more normal setting on the sub level control and that it sounds "fantastic", there's no need to do anything, including spending money on any expensive magic power conditioning items.
Exactly. Hammer, SVS even notes that receivers give out sub line-out distortion at higher levels and recommends to set the sub volume at -5 in the receiver, then calibrate for that.
So, I'd pass that recommendation to you, set the "woofer" level to -5 and that will reduce the distortion hum you are getting.
