Hi. I have a Sony surround package and the subwoofer hums. Even if it is connected to a wall in another room, not to the A/V receiver. Should I simply replace it with another powered sub, or is there a fix?
 
Thanks.

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

Also look into what we refer to as a "ground loop" problem. One of your components may not be properly grounded and as a result its producing a hum. Go to the hardware store, get a 3 prong to 2 prong ground lift and see if that solves the problem.
 
Da Worfster