Thanks Alan, but I'm with ldrew. The LF noise went away as soon as I rebooted the EP500. If it returns I'll try to "disconnect" the EP500 as suggested to see if that also stops it and report back - but so far so good!
Nathan

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