The amp is not causing the distortion in your back speakers. Your factory headunit is limiting your options to improve your sound quality. I would recommend getting a new headunit with a built in crossovers to seperate the highs and lows. Right now I have a pioneer headunit which seperates the highs and mids to come out of my interior speakers and the lows come out of my sub. One thing you can do right now, which is ghetto but it should help out alittle is to lower your bass on the headunit so you'll only have bass come out of your sub. This will help save your interior speakers from blowing.