Well, if you don't mind losing 1-cubic foot of space in the trunk, get a small 10-inch sub with a 200-300 watt amp. I have a really, ultra shitty sub/box with a quality 180-watt amp and it's fucking awesome. I mean real ass kicking air-twitching awesome. For real dog, it's the shiznit yo.

Kidding aside, it thumps like stupid. I mounted the amp on the box and can even remove the whole thing in 2 seconds if I end up needing the space, so to me it's the perfect solution. The rest of the speakers are stock, and the head unit is some Panasonic thingy with a seperate sub-out so I can tweak it specifically for CDs that need some boost. The amp cost me $250, the sub/box was $100 LOLOLOL-cheap.

Edit: HOly fuck I can't read, you even said you don't want a box in the back. More coffee, please. Yup, some decent 6x9's with an amp would also sound good.