Suburban SubWoofer?

Hi, new to the forum.  Have an '01 Suburban, and replaced the door speakers with Infinity 6002si coax's.  Sound is tinny at low volume, but OK when it's louder.  I'm thinking the new speakers are more efficient than the factory 8" sub.  Is it worth it to replace the factory sub?  Would like to keep the factory HU because of OnStar.  This is not the Bose system.

Crutchfield recommended the MTX Thunder4500 T4508-04 8" 4-ohm Component Subwoofer, but also notes custom mounting required for factory location.  I have no problem doing the work, but don't want to waste my time if it' s not going to help the sound.  I don't want to go with a separate sub box, since I fold the seats down often to carry plywood, lumber, etc.

Suggestions?

Fiberglass an enclosure in the side panel like this - http://www.loslabs.com/gallery_images/100_1467.JPG

This guy also did one with 2 qty MTX T5508-04 woofer w/MTX Thunder 251D amp tied in to the factory Bose system. (sorry, I don't have those pics...) Keep the woofers close to the floor or backwall for best response. Pretty stealth too, if you hide your amp...

MHO - I would use an aftermarket amp. Bose amps can be a little 'picky' when hooked to aftermarket speakers(especially multiple drivers). In the older Bose systems, you can tap in before or after the sub amp(which should be under the back seat, on the floor, if memory serves...).

Apline is correct - Ai Research makes the Q-Logic stuff, but they don't make the backdoor one for your truck. They make a single 8"  in the center console. JL makes one with 1 single 10" if you have bucket seats & center console.