Stock stereo vrs aftermarket (both with subs added)
The stereo place I dealt with installed an amp with stock unit and gave me an additional adjustable gain switch with it. It was finicky. I had to have amp gain real low, and if I turned up sub gain, the sub amp would shut down. I had to re-start the car to re-set it. Finally I got a real aftermarket CD player. I lost steering wheel controls for it though.
Too bad you don't have a wagon like my 2006. The subs I DJ with I hook up in the car (Dual purpose and storage in the car covered with grill cloth and dark tinted windows). Everything vibrates.