I really like the direction you are going. I'm planning my new system now and I'm hoping your results are awesome so I can copy off of you.
I like the idea of a smaller sub for both space and cost reasons. And, I've done the 15" sub that takes up the whole trunk.
What are you running for components? I looked at a handful of options. Everything from Focal to MbQuarts. Hard to decide based on reviews. After seeing some of the geniuses at car shows, not sure I trust their "ears" when it comes to the front stage.