I just wanted to follow up on this:
After doing research for quite some time, I chose to just JL Audio 10 inch IB (Infinite Baffle) subwoofer speakers. For my installation, I used 2 JLA 10IB4 subs, mounted free air in the rear wall of the trunk facing forward into the back of the rear seats. Also, a 300 watt Phoenix Gold amp. The trunk is a 5 cubic foot sealed space separate from the main cabin. (on a convertible, the main cabin can vary between a sealed volume and an infinite volume ;-). I had to do quite a bit of additional work isolating the spare tire compartment from the main cabin because the spare compartment is built deeply.
So at the end of the day, I'm able to take out my Bazooka amplified tube which takes up all the trunk space. That should give an idea of how small the trunk is on my convertible, my old 8" tube sub takes up almost all the space in it. :-( However now it is replaced with 2 10"s :-), and a get my trunk back.
How does it sound? It sounds wonderful, very smooth with a good amount of slap. My system just went up the scale by quite a degree. With the gains adjusted too high, it can make the whole car blurry. With the gains properly calibrated, it can make you just cry! It's very musical!! I ran the iasca cd through its paces and somehow it improved the stereo separation -- so the sound staging is incredibly clean. I can't quite explain why or how, maybe the 24db crossover in the phoenix gold had something to do with this.
Anyway for those with my car (basically no one ;-) I highly recommend the free air install.