Very good, maj; now you can turn all the power circuits back on and re-plug the 73 things which you unplugged! Seriously, it's again an illustration of fine Axiom customer service to replace a unit which clearly wasn't being merely affected by a ground loop.
When you get the new sub, the chassis screw should probably be left in, since as Chris correctly pointed out in his excellent description of its function, it can have some beneficial effect if hum isn't present with it in.
On the question about the sub cable alone(not even connected to the receiver)causing more hum, I was just curious if it did seem to be acting as an antenna for airborne electrical interference, as I'd speculated. Of course, the point is now moot. Enjoy.
