[quote name='pete.young' post='11903' date='Jun 4 2007, 04:08 PM']I'd like to ask what the significance of the Vas is, but I'm not sure I'd understand the answer![/quote]
Vas represents the equivalent volume of air to have the same spring compliance as the woofer. Basically the higher the Vas, the larger the cab needs to be to get full bass response (i.e. resonant frequency of speaker in cabinet equal to resonant frequency of speaker in free air).
The reason that few bass cabs have the lows they claim is that the speakers have too high Vas for the cab they're in. To get a lower Vas without giving up sensitivity is difficult, hence the "loud, low, small, pick any two" law of speaker design.