I'm in the same boat. I removed the ground screw, and while the hum was considerably less, it's still there. I haven't tried a ground isolator yet. I'm upgrading to an amp/preamp combo this month, and I'm hoping that, as a side benefit, using an XLR connection to the sub will eliminate the hum. I'm not sure on your next course of action, but a ground isolator might be something to look at. Again, I defer to ANYONE else's opinion here.

http://www.axiomaudio.com/groundisolator.html