Wgen you say "turn up past half way," do you mean the volume or the gain? On all amps turning up the gain will increase distortion, I've got an old Fender that sounds like i connected a fuzz box if i turn the gain more than half way up. Try setting the gain low and the master volume up. Back off on the EQ or tone controls too, boosting anything more than about two thirds of maximum will increase distortion. Another thing, does the amp do this with all guitars or just one? If it's just one you might turn down the volume on the guitar or possibly need to lower the pickups, if it's a problem with any guitar you need to get the amp checked.