The HK sub and amp will make it sound better, if you just want it to sound better. I guess that's your best bet if you want to keeping everthing with the stock set up. But if your looking for great sound from your stock setup as the foundation, your not likely gonna get it. You could always run your factory deck to a line out to a better amp(better meaning Alpine, RF, Soundstream, etc etc, not BMW) and then run upgraded speakers and sub from that amp but if you want to go custom go full custom like jjjjjjj said. Hope this helps, keep us posted on what you do.