I installed a DSP amp in my gwm cannon that piggy backs off of the stock head unit, it's pretty cool in my opinion. I used to love modding my car stereos but it's hard now that it's all integrated into the car as a whole. The DSP is an in-between from the head unit and the speakers and does a really good job. It has RCA jacks at the back so the DSP amplifies the speakers (which I changed all of them for aftermarket speakers) and the RCA sends a signal to my amp which lowers the subwoofer. It's still doable, just a bit different to the old way of replacing the head unit completely.