Nice work!
Regarding the subwoofer, physics is working against you trying to get heavy bass in a convertible. A soft top won't really hold the sound pressure, so you have no cabin gain to help you out. To scale things up, think about concerts at indoor venues vs. outdoor venues. The only way to "feel" the music at an outdoor concert is to be in the first few rows near the speakers. Inside a club, you'll get blown away - and they're pushing half as much power indoors (and two orders of magnitude more than you'll ever want to use in car audio).