VAS is the stiffness of the suspension, there is a definition here:
http://www.hometheatershack.com/foru...arameters.html
Since the recommended enclosure size for an IB is 10 times the VAS of the driver, it would take a reduced amount of input power to reach the Xmax of the driver in an enclosure that size. Reduced power input means reduced SPL output. Since low end bass is all about moving air, (displacement), the use of multiple high Xmax subs is required to move the air necessary to produce the required low end SPL. You could use the subs from SVS, Cerwin Vega etc, if you had enough of them to move the required air to produce the SPL.