you have the EX, you should have the door tweeters...

simply upgrading your speakers from the stock paper cones will make a world of difference. you can upgrade the in-door tweeters too, which will help with the soundstage.

as far as subs go, i have a single 12 and it's more than enough. my friend's got 2 10s (1 on each side) and it sounds really good. i guess that's the way to go. you could pull out the stock trunk wall carpeting and try to make some custom enclosure on each side if you're worried about trunk space.