I used the LC2i on my install and it works like a charm. It compensates for the bass roll off at higher volumes from the factory head unit. It is a little tricky to tune it, but it sounds great when you get it right. They are like 99 bucks or so online. I wouldnt change the door speakers unless you are going to amp them. Most aftermarkets wont sound right off the low powered head unit.I dont know if you have stumbled across my build thread yet, but i have some good info on the lc2 there in some of the comments:
Hope this helps a little, what sub setup do you have?