Well, it seems that I have more questions than answers. I think I've decided to go with the JL Audio 6" subwoofer to replace the 6" midbass driver that came stock in the car. The existing enclosure is a fiberglass/resin (?) about 28" wide and about 16" tall and deep with curves (UGH!). I guess I can use AutoCad to calculate the area, but here's the catch: JL Audio only recommends an enclosure of much smaller than this is. I know that much. In fact, I believe the area is sufficiently bigger than their recommended size for a ported enclosure. I'm going to give the enclosure a shot, but would I mess up the stock enclosure by turning it into a ported one? With only a 6" subwoofer and a larger than recommended enclosure, would I even notice the increased volume?
Thanks for the help!