Those other subs like jbl probly have smaller xmax, and on a little bit of wattage seems like they won't produce much compression. I have actually read a thread where a guy said that the power you are running is also vital to calculation.
What would be awesome is to create a sub that had a nitrogen chamber to recycle nitrogen to liquid over and over, or get some system tthat atleast makes a gas cold like a good fride style. Would make subs alot more efficient! Could maybe spend less on amps..