Well if it's more bass you want, then I'd suggest getting a sub and amp combo.
If you don't know anything about ICE, the best thing to do is to buy a sub/amp/box combo off caraudiodirect.com, or caraudiocentre.co.uk,
And then research on how exactly to fit it, or take it to a specialist!
Here's a good place to read up stuff on
http://www.talkaudio.co.uk/vbb/index.php
Adding 6X9s to your system as it sounds, would be a waste of money and effort IMO. It's not about how many speakers you can put it in your car, it's about the quality of the ones you have, and the frequency ranges they produce.
An ideal setup would need to cover all frequencies from low to high, which is why it's so good to simply have decent components covering your high and mid frequencies in the front, and a decent sub covering your low frequencies in the back.
Your current sound system in your car is alright as probably made up of 4 coxials, or maybe 2 coxials and 2 components (types of speakers), I'm not sure. But what ever it is it will cover the mid well and probably be arite for treble too, a sub will add bass and complete it nicely.
all 6X9s will do is add to the mid further (which will cloud your current speakers), and maybe add a little bit more bass.
Have a look here for sub packages:
http://www.caraudiocentre.co.uk/categor ... _c-482.htm
What's your budget?
