Both the stock speakers and amp are weak
Replace the OEM hi fi amp , if that's what you have, with a Jl xd600/6(the oem amp is the weakest link)
use a Technic harness. Replace the front door and under seat speakers next
with the ones commonly listed in this thread.
(Replacing the speakers alone won't get you far you may even go backwards the amp is really weak)
http://www.e90post.com/forums/showthread.php?t=641323
Too many people believe a sub is the solution for all problems. The sub is for 50 hz down(all true subs are) the under seats are for up to around 200 hz and a little higher if you use Jehnerts.
The under seats carry midbass and lower midrange and there location also means they are more effective in generating all but the lowest frequency. Finding a 4 inch that will extend far enough to bridge the gap to the under seats requires money. The Morels seem to be the best at it.
MusicarNW can sell you proven systems that work in this
car but there not cheap. There are no dead cheap solutions.
By the way where in the US are you located?