Yes, the 6x9's probably should have gone in the front. Before you change it, sit in the back seat and listen for a while. Is that enough bass for you? Play with the HU settings too. If not enough for your tastes, then a sub/amp will do the trick. Pick a budget and some goals (like how much trunk space, how loud, best quality). We'll help some more.
Oh, you might want to try dynomating the doors. Seems to help the bass response as well. But go with off brand to save a lot of money.
