Linearity, excursion, power handling.
Linearity is the ability of the driver to move in and out along exactly the same path. The better the linearity, the more accurate the sound prduction.
Excursion is how much the driver can move in and out. Your driver will have less than 10mm of excursion and may well have as little as 5 or 6mm. The more excursion a driver has, the more air it can move and the louder/lower it can go. There are now drivers with 30+mm or excuresion.
Power handling. The lower the note you want to reproduce, the more power it takes. 130w is fine for anything in the 40Hz range, but is probably not enough juice for stuff in the 20s unless you have a very efficient (read large) sub design. Drivers being inefficient devices produce a lot of heat. The better a driver is at dissipating heat, the more power it can take (within its excursion limits).
Mine seems to get down to 25HZ and rumbles pretty well.
Have you measured your sub's output? If you do, I think you will be surprised.
