I'm basically did the same thing as JDouglas, except I don't have speakers in the doors. I put some rockford fosgate 6x9 in the back, and some kicker 4x6 in the front. I faded some to the rear some because those 4x6 are really treble heavy. I wasn't going for anything pro, but with the rear doors sound deadened and a $90 head unit in there, it does its job.
You could probably fit larger speakers up front as long as they are shallow. You might need to completely remove the dash to cut it properly. Also, I'd probably say more bass up in the dash is just going to rattle things around. Just wire up a 10 inch sub somewhere if you want bass.
