This info comes from a review by Crutchfield's John Pollard
JBL's GT-BassPro12 powered subwoofer adds a blast of bass to your vehicle so you can really feel your music's impact as you drive. JBL designed this bass system to shine in the car environment, tuning it for tight, responsive low notes.
The GT-BassPro12 enclosure features a JBL 12" sub that really brings the thunder with its polypropylene cone and hi-roll rubber surround. The built-in 150-watt amplifier is matched up perfectly to the sub, so it won't wimp out when it's time to rock. Use an amp wiring kit (not included) to power up the GT-BassPro12, and you've turned your car into a concert hall.