Great stuff guys.
...So my solution was to use the rearspakers as subwoofers. (rearseats arent really used, since there are no hobbits here)
I replaced the doorspeakers with a good 13cm component set (an old pair of Pioneer TS-C series), and for the rearspeakers I got hold of a 17cm component set form Pioneers TS-A series. This has strong focus on the bass, but the tweeters are not much of a thrill, but I didn't need them anyway- woofers mounted in the car, tweeters in the bin.
So the trick is to get a headunit which can: A) play the rearspeakers as subwoofers B) separate the subwoofer-frequencies from the front speakers.
Pioneer DEH-8400 can do this, but probably many other headunit of various brands can do this, and of course many other speakers are capable of acting as subwooferes ( JBL GTO series, Focal, Morel....you name it)
Shoot if you have any questions. I know a thing or two about stereo