Thanks for the suggestions. My symptoms sounded a lot like mdrew's as it also took me awhile to work out the noise was from the sub. I've tried "rebooting" the EP500 and that seems to have worked for now - will keep an eye (or ear on it).
Thanks
Nathan

HT:M80,VP150,QS8,EP500 Outside:4xM3 Office:AudioBytes, Rumpus: M60,VP150,M22 Portable:2 x Airs