My suggestion would be to proceed step by step. Start by adding a sub amp amp and sub woofer. As NCX suggested JL 10" Stealthbox/amp or BSW 8" Alumapro/amp are great choices. You will immediately feel a big difference. Much more profound bass which will significantly improve the listening experience. A lot of people are satisfied by this simple improvement. If you want more then go for BSW stage 1 as a second step.