yeah, exactly what they said, an HDD is a magnetic storage device, data is stored via magnetization of a rotating magnetically coated surface, if you're not careful, the magnetic flux that surrounds a subwoofer, when in operation, could cause some data corruption in the long run, i wouldn't really advise it...

plus everyone's heard of how delicate the 360's dvd lens is, so next time you crank up the volume and fire off a boom shot in gears of war, the bass from that might be enough to boom shot your dvd drive as well...