Since I don't consider the M200's to have a hiss problem (the pair driving my Studio/60's certainly don't), I did some digging through threads to see what sorts of hiss problems have cropped up here in the M200 forum. Many of the cases I've seen have been the result of ground loops or other equipment (space heater, dimmer) on the same circuit. As to why an M200 would be affected by a ground loop that didn't affect a 755, I'm not sure.