Larger diameter speakers do not always mean lower frequency response. A great deal of that depends on how well the box size and design is matched to the specs of the woofer. I've had a JL Audio 10W3 in a box custom built per JL's suggestions and it yielded much better frequency response as well as being extremely hard hitting than some other 12's and 15's in a 'standard fit' box that, again, was not designed for that subwoofer to begin with.

Soldier [X], what sort of power will you be running to your subs. That is an important consideration as well.

I've been a fan of, and have used 10 inch subs since 1988 and really love the mix of compactness and performance. Sure, you won't break any SPL records with one 10, but then again, hopefully everyone is not still "about" going deaf at 25 years of age

Don't get me wrong, with just one 10" sub and 100 watts or more power (depending on the subs requirements), you can have very hard hitting and low end bass in your car, be it a Civic or SUV; AND you don't have to sacrafice your trunk space.