Mike, just for the heck of it, try a good powerbar / surge suppressor thingie.

Usually, these ground hums are related to the interaction of different components' non-common grounding, or the issues of, say, your TV cable having a different ground source.... but that doesn't mean that it can't happen internally within a component. It's worth a quick try in case the receiver itself is picking up something from a refrigerator, dimmer circuit, or a neighbor.

::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::