Well, I'm not going try and bring down sundown or anything, because I do respect them a lot. However, I don't believe in the port per cube.
I go the route of port velocity and determining how much air is being moved and keeping it going under a controllable velocity, which with modern subs equates to some very large port per cubes.
This is a quick calculator to get you in the right direction.
http://www.carstereo.com/help/Articles.cfm?id=31
This will lower air velocity, but you are still going to be moving a hell of a lot of air, especially if you tune low.
Also, if there is someone reading this that knows a lot more about moving air than I, then speak up. I'm a SQ guy that knows a little about aerodynamics.