How hot are you recording? So you aren't clipping at the mic, so the pre isn't turned up too much? Also you do have your master fader on your mix right. When I first got started I really didn't even know what a master track was and as I'd add more and more tracks I'd start getting more and more distortion. Finally I learned just to set up a master fader first and turn it down. I know you probably already know this but just trying to help.