More likely their are some bad components on that channel, like a bad capacitor, etc., that is impacting the sound quality.
Hitting the MT120 with too much level won't do any real damage to the unit, like it would if you played music too loud in your car and blew the speakers. The MT120 would just give you distortion if the level was too hot.
Don't overlook cable issues as well, if you're having an issue at the input stage. I had a situation a couple weeks ago where I thought I had a bad mixer channel that turned out to be a bad insert cable.
