I replaced my door speakers all round with infinity co-axial speakers, pointed the central tweeter unit upwards and disconnected the built in tweeters as a lot of people had mentioned doing this and that it sounded OK. In my case I thought it sounded pretty bad, having the high frequencies coming from near the floor just didn't seem to work, especially with my (or a passengers leg in the way), the highs just seemed to disappear.
Anyway, to cut a long story short I reconnected the the built in door tweeters all on all four doors but put an in-line high-pass crossover to filter the bass frequencies (as I believe there's nothing filtering them as standard, or if there is it doesn't seem to be very good). Since I put the in-line filters in the sound is terrific, really balanced, crisp and clear and thanks to the new door speakers, loads of deep bass
I used these filters, really cheap and you get 2 per pack so only need 2 packs to do all four doors:
(Not allowed to post URL's yet so search for this on Amazon.com - not the UK site):
Scosche-XOHP-Xohp-High-Pass-Crossover
The only
problem is I had to get a mate in the US to order them and send them over as I couldn't find them anywhere in the UK.
Hope this helps.