I bet that these new amps by Motion Sound, the KP-200 and the KP-100 would help this problem because they are stereo and have a spatial expander to expand the stereo signal. I prefer to go mono on my PSR2000 rather than the right channel, I find this sounds better for me. I do occasionally hear that phase shifter sound. I'm sure the problem would be worse if I performed more modern styles.

Beakybird