i know i'll get flamed - but the factory MTX subs w/ 200 watt amp is DESIGNED to BALANCE out the Boston Acoustic system. 200 watts doesn't sound like much, but at the impeadance level it runs it's comparible to a 400 watt amp. it's $780 shipped thru gmpartsdirect.com. little pricey i think, but i've read 6 folks reviews and they ALL raved about it.
your dealer can install it and not only will it not void ur warranty, it will be covered by ur 3/36 bumper to bumper warranty.
if you don't have an SS and the Boston Accoustic system, you need lots more than subs.
my opinion........
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IOM 2SSRS LS3 R6P - VIN 7553 Delivered 5/22/2009 Kooks, Corsa and Roto-Fab - SOLD 9/2010
VR 2SSRS LS3 - Borla, SLP CAI