Hmmmm Steve,
Interesting...do you have to get real close to the sub to hear it, or can it be heard from the seating area? I used to have a similar problem when I had my subs plugged into a different outlet in the room on a different circuit from my other stuff. I think I could even hear the sound coming from the other speakers.
As soon as I plugged the sub into a different outlet on the same HT circuit it went away. Does your 500 have a ground screw on the back amp plate, if so you could try removing that and see if that helps, but that is normally for ground hummm.
The only other option would be to call the Axiom tech 800 line, Brent, JC, and the others answer that until the evening hours...
