The hum could be from many things -
It may not be from the actual mains itself, the subwoofer can still pickup interference from other sources.
-EMI/RFI being picked up by the input signal lead
-EMI/RFI/mains hum being picked up by the power lead to the subwoofer
-The subwoofer itself being in close proximity to electrically noisy devices (e.g., el cheapo $30 DVD players, heaters, fans, motors, fridges, microwaves)
What you can try -
-Use good quality interconnects - no need to fork out several hundred for THX spec cables, but don't use crappy $4 bargain basement leads.
-Make sure the power lead/interconnects don't run near noisy appliances, or are coiled around other power leads
-Try getting a powerboard with EMI/RFI filtering - basic ones will set you back $15