Whenever given the option it is always best to go with multiple subs. I would opt for the pair of supercube's I if it would cost the same as the Velo DD-12. If your lucky you can get great bass from a single sub but it is all dependent on the room. It is much easier to get good to great bass with 2 or more subs.
SVS subs are also very good. I owned a pair of SVS Ultra's that were so powerful they actually knocked a big picture off the wall in my last home. There was one room in my old home that actually shook violently when playing action movies and at sane levels to boot. I really liked those SVS Ultras but they were the tube variety that looked like tiger scratching posts. They do make really good looking box subs now and there customer service is the best I have ever encountered.
I like the idea of the eq for the subs but you could add that later if need be from company's like Speaker City that sells the SOS, AV123 that sells the R-DES and Velodyne is coming out with an outboard eq that is the same as in the DD series that you could use on any subwoofer.
An eq is only necessary if you have peaks that cannot be corrected by placement. I tried a R-DES from av123.com but found that I was able to reduce the offending peak by just moving one of my 3 subs less than a foot from the rear and side wall.
Good luck
