I may have to play with this...
If its offensive, it can be reduced by using cavity shielding and shielded wire, but its a bit tricky, and you have to make sure you don't create ground loops. When I did my first Les Paul, I used the old style wire with the braided steel cover and grounded it in about 4 different places. IT was awful, much worse than before.
I noticed that when the electronics (pots) are near my body or if I hold the guitar away from me and put my hand near the electronics, the buzz increases. Other than that, I think it's the normal hum.
EDIT: I'm not going to rewire my current guitar with shielded wires, just try and shield the cavity.
....Now about that tone pot.
RE: Power supply - I was referring to my laptop, the Mbox is powered via USB. Just thinking that if the laptop power supply was a 3 prong, maybe it would help...