As for Airport Express, I have it connected to a surge protector close to the printer, stereo and speakers. When the usb printer cable is connected to AE, I can't connect the minijack for Itunes because I get a soft but constant hi pitched distortion coming from the stereo (also a very basic but good 10 year old system: no toslink or the like). I have to remove printer cable to play Itunes (on the Aluminum Powerbook).