Hi Danny,
I have a KSW10 subwoofer and it has had a low hum from day one. I have gone through all the possible solutions (changing the plug end, unplugging and replugging connections etc...) but it is quite clearly not a ground problem but a problem with the internal amplifier in the KSW Subwoofer range. I am afraid you might have to live with it and Klipsch should use some better amp shielding in their future range of subwoofers.
Italo