I was thinking about the area of the driver in the SVS Ultra 13 vs the 4 Axiom 6.5 inch bass drivers. The area of the SVS driver is pi/4 * 13^2 = 132.7 (approximately). The area of the 4 Axiom drivers is 4*(pi/4)*6.5^2 = 132.7 (approximately) (I thought is was kind of a funny coincidence when the numbers came out exactly the same).
Cool! I'm to lazy to figure things like that out. I does show why the M80 has such good bass. Though there is more to a good driver than radiating area, it sure is a good start.