I thought I would throw this suggestion for those looking for a quick and easy subwoofer upgrade option.
The subwoofer linked below easily fits in the back seat and is nearly invisible from the outside (put it in a rear seat facing downward). It does not require a lot of power and a small amp can easily be hidden in the trunk. The total cost should be around $500-700 with installation. Only drawback is that it needs to be removed if you want to put the rear seat down.
This setup prevents the subwoofer from shaking the rear panels (a big problem when it was in the trunk) and greatly improves the sound quality. It will not win any competitions, but it is the best option I have found that does not kill the trunk space and uses the useless rear seats.
The only way anyone would know the subwoofer or amplifier are in the car would be by sitting in it or by blasting the stereo.
Kenwood Excelon 10 Mobile Subwoofer Enclosure - P-XW1000B
http://www.google.com/shopping/produ...ed=0CDIQ8wIwAA