phil, turn the volume down. If you still hear the distortion it is most likely your cartridge miss-tracking in which case you have to go over your tone arm set up. Koetsu's are not the best tracking cartridges. It could also be the cartridge overloading the phono amp which I doubt unless there is perhaps a bad tube or the phone amp overloading the preamp. If the distortion goes away with lowering the volume then you are most likely clipping your amp. But you said the volume was not that high so I have a hard time with that theory.