Don't fall in the bigger is better trap, it is POWER that makes bass not the size of the woofer, my 6 1/2's make a LOT of bass over 80hz the doors shake and they are damped, the mirrors bounce, but they are getting a solid 150 watts to each and another 150 to the huge tweeters, I have to boost the signal from the headunit to the amp to get the full 150. It is the quailty not the size, I have used other larger speakers that were much cheaper they sucked. There is a reason Image Dynamics keeps winning the world championships, but they are EXPENSIVE!
A three way system would be nice but in our small cars not sure you need it and that you would not end up with hot spots, where would you put the mid's? I just have a single shallow 10" sub in the factory location and it thumps. I plan on swapping the MB Quart for an Image Dynamics that will be deeper and require and very custom box in the spring, I spoke with ID on which 10" to use the 500 watts I have now should be plenty.