The box sizes and tuning we post on the AQ/DD web sites are NOT set in stone, they are STARTING POINTS to work from. We test our subs in many different box sizes and tuning in many different vehicles during the developement of the subs.
One box size and tuning is not for everyone but if you use those boxes and tuning you will have a a good combo of sq/spl/output which is what most of our customers want. If you tune too low your upper bass extension will suffer, if you tune too high your low end will suffer. We build the subs with very strong suspensions/motors in order to play a little below box tuning without unloading the sub and still have good high bass extension.
34-35hz tuning is about as low as I would tune and 40-45 hz on the upper end for those guys who want max output (spl). 38hz gives a very good combo of both.