You also might want to try playing with the crossover on the amp. Depending on where you have the speakers crossed over, you might get more lower mid-bass out of them. Low bass wont happen without a sub.

Depending on how loud you want the system to go, you "might" be able to get away with a cap on the speaker....something around 100Hz at a 6 db slope should work...something like a "bass blocker". This route is good if you can tell when the speakers are distorting, and back off a bit. If you are abusive, I would not suggest this. Use the amp crossovers then, with a 12db (or greater) slope.