I have another guess here. Some phono amps do not have a subsonic filter built-in and the extreme low frequency caused by the warp may have overloaded the input of your preamp. To test this theory you can either hook in a subsonic filter or hook in an attenuator to reduce the input to the preamp and see if the distortion goes away.
Again, this is just my guess without seeing your problem with my own eyes.
Again, this is just my guess without seeing your problem with my own eyes.