Have you tried their website:
dualav.com?
http://www.dualav.com/carsubs/index.html
here's the user manual for the DS12 (their basic sub):
http://www.dualav.com/i/product/manual/dssubs.pdf
page 8 of 12:
Good Bass = .75 cu ft
Better Bass = 1.0 cu ft
Best Bass = 1.25 cu ft
I'd hate to be mean about it, but it took me a whole 14 seconds to find this info. You've got over 350 posts here, so I'm sure you've seen people get flamed for not searching...