The factory sub sounds like crap on the rear deck. Just messing around the other day, I put the sub on the trunk floor firing backwards. The deep bass response that was never heard before suddenly came to life. If this were a ported enclosure, it would sound fine in the trunk, firing backwards, and mounted close to the rear seat back. In my past experience, I've found that sealed enclosures with rear firing subs, sound better closer to the back of the car. Vise versa with a ported enclosure. I say this becasue with the sealed factory sub sitting closest to the rear seat on the floor, sounded great when I popped the trunk open a crack.
BTW, anyone have an extra sub amp they'd like to get rid of, along with another factory sub box? I've got wheels turning in my head for some modifications, but still intend using the factory stuff. Just for a little fun project.