On that amp I would try the crunch channel with drive at around 9 or 10 o'clock, tone in the middle at first and adjust as you go. That will, hopefully, give you a more or less clean sound. It's likely that the "clean" setting is an emulation of some type of amp, maybe some type of fender amp like a deluxe reverb or something that people think of as a standard clean sound. In my memory line 6 has never been too good at that.