Hum comes from a difference in ground potential - ie: the sub "ground" may be floating at
*WARNING - HIDEOUS OVERSIMPLIFICATION*
2 volts, while the amp "ground" is at .1V for whatever reasons - different house paths to ground, but that's a book in itself... it's this differential that creates hum.
One way of killing the hum is by breaking the AC ground connection on the sub, so it's AC ground is determined by the amp AC ground through the signal return (let's assume a coaxial interconnect and call it the shield) - this works because the sub will get its ground "level" from the amp, the problem is the shield isn't heavy enough (at least 16ga.) to provide the adequate shock protection in case of a short - will take a dangerous amount of time to trip a breaker, or worse, may act as a fusible link and simply burn out while you stand there doing the 115v funky chicken in a pool of your own urine.
The safer way is to "lift" the signal ground, ie: connect the shield only on one side of the cable, it breaks the ground loop and allows the safety ground to remain intact on the sub for shock prevention.
Quit looking at me like that, audio isn't passed on the shield (but video is - don't try this with a video cable of any type - whether composite or component!)
Bren R.
