Thats fair enough if thats the case. However i would recommend looking at a Gladen woofer for your underseats. The link
http://www.gladen.com/compo_coax.html#one scroll down to the second last one.
I heard the set a few weeks ago in another members car and the woofers sounded amazing. Played alot lower than my jehnerts, and most people would think there's a sub in the car. They also sounded better than Earthquakes. It sounds like you want this so try and find out if you can just get the 8" woofer and crossover on its own.