Excellent ground loop (or unequal induced current - we are not sure) discussions at this meeting today, while consuming prodigious amount of beer, wine and chips and salsa..
We concluded that - generally speaking - it is fair to aspire to connect any Sunfire sub to a different phase from your pre and power amp, ideally a separate circuit from the next county over. If you do that and get decent interconnects, and pray intensely for several weeks, hum may drop to very low levels. Otherwise let it hum away for all its worth and hope for no one to notice.
Pepe