I agree with chroot. Get a custom fiberglass box made for the trunk well and put in one twelve inch sub (I have this set up). Don't settle for a ten inch sub, it physically cannot compare, Don't have to make the box too big, 0.8 cu ft gives tight bass. Also, pick up a good used amp off eBay, save yourself over the cost of new.

 

5.25" will give better bass. But there's no SUBsitute for a subwoofer. One 10" really is a enough for most people.