I have a Vandersteen powered subwoofer with a constant 60 cycle hum. I might have a grounding problem because we have a 50 year old house with the 2 prong outlets. Is there a way to defeat this humming under these circumstances? I've tried "grounding" to the outlet using the mounting screw and this only somewhat quieted it more than the connection that I ran from the sub grounding screw to the shielded portion of the speaker wire. I tried running a long wire from the ground connection to a light switch and this made it worse, almost like this wire was acting as an antenna to surrounding RF from the house wiring. The main speakers are Martin Logan's and are also powered; these are quiet. Any suggestions would be appreciated.