Repeater/tone changes when preamp changes

[AALev123@aol.com]

I am experiencing  a change in the tone of a looped passage when I change the patch of my preamp. For example, I lay down a 4 bar clean chorus part and it loops back fine. However, I change the preset on my preamp to a distorted patch the clean looped passage keeps playing but the tonal characteristics completely change. My preamp(digitech 2120) is before the repeater in the signal chain. I am using a DMC GCX switcher and ground control. The repeater is in loop 8 all by itself. Shouldn't the new stuff just get mixed in with the old already looping part? Why would changing patches on my preamp have any affect on the tone of the already recorded looping part. P.S. Digital Music Corp said it is not the GCX. Thanks