Ouch! I decided to go the cheap route and install this under the driver's seat once my 11 gets here: Amazon.com: Boss Audio BASS600 8-Inch Low-Profile Amplified Subwoofer (Single): Electronics
Not the most powerful little thing, but should add a little bump to my tunes.
EDIT: Wow I see it's listed at $286 now on Amazon. Mine was a Christmas gift (that I picked out ) and was only $100. Sorry for the misleading info haha.