Replacement Underseat Woofers
There are a lot of great threads on this subject if you do a search. Whether you like the Earthquake subwoofers a lot or other speakers that are more mid woofers depends on what you call music. I have 8" Jehnerts (German speakers) in mine and am very happy with the result. I do have to say that the Jehnerts seem pretty expensive for what you are getting, when you hold it in your hand.
Have you replaced the 4" speakers in your doors and put tweeters in the sails yet? If you are looking for the most value for money, this is a better first move, in my opinion.