Because the box is plastic, there is a chance of it rattling. What that means is that if the box is moving around, there is less vibration noise, and less of it delivering the "push". Sort of like trying to punch something while you're being shook around. Now, it's not that drastic, but that's the theory. You can get the sound deadening material in sheets or in a can of spray. All it is really, is a thick rubberzied material to add weight to the thin and vibrating plastic. Car audio people do their whole cars that way in order to kill road noise. You can get it at car audio shops, some electronics stores like Circuit city, Best buy and Frys, and sometimes car parts shops like Pep Boys.
Sometimes the subs aren't that good, but the new t4000 are supposed to be pretty good. If you want a different sub, you just have to find out the box depth, take about 2 or three inches off that, and that's how deep the sub can be at max. Lots of people do that as well. Because the box is small, there is little air space, so you're getting a very tight bass. If you get another sub in it, make sure it's one that only needs a tiny bit of air volume.
If you have a '93, there won't be too much space to fit a slim box, I don't think. With that much space, it seems less than the stealth box. Chevy Full Size trucks have boxes like that, but those have plenty of room.
