Most subs require less space if they are used in a sealed enclosure. I'm sure the factory sub's Thiel parameters were spot on with the matched enclosure as well. Also, the factory speakers are 2-ohm, so they require less wattage for higher volumes.

 

I've been watching this thread for a bit and figured it was finally time to chime in Ok as for the aftermarket sub in the factory box, I personally think it's a fantastic idea! I did it myself and all I can say is "WOW what an improvement!" However, you do have to choose a sub with the correct parameters for the box which leads to the question of all todays subs saying they can work in a .3 cu.ft. enclosure. Yes and no. In a lot of cases .3 cu.ft. is the bare minimum it will work in. Optimum enclosure volume could be .75 or whatever. This will have a large affect on how an aftermarket sub will sound in the factory box. As for stuffing a 10" sub in there I say most definately yes! However, I would not recomend sticking one in the factory box (too small). There is a ton of room back there. If you were creative and can handle building a fiberglass enclosure then I see no reason why couldnt squeeze at least a 1 cu.ft. box out of that space. Your only limitation would be the mounting depth of your sub (flipping it backwards is one very easy way to overcome this). As for flipping it backwards, this has no affect on the driver accept for now you are looking a magnet instead of a cone. The reason a speaker works is the back and forth motion creating pressure waves, not some magical thing that happens on the front of the speaker Finally I want to comment on the factory amp stuff. I believe it is 280 watts (or whatever they rate it). Size doesn't matter here. It is important to remember that this amp is "purpose built" for this application and to fit right there on that box. It was made specifically to run those 5 exact speakers and fit in that tiny spot per GM (or whoevers) specs. The reason aftermarket amps are soo big and pretty is the WOW factor. If you saw our factory Poineer amp sitting next to some cool looking Rockford Fosgate amp in the store and both were the same price, which one would you buy? It's all about dollars in the aftermarket and rightfully so. They are in business to make money. I hope some of this helps you guys