I had an svs sb12-plus, which got replaced with the sb12nsd. I really liked my svs sub. The build quality was fantastic and I liked the fact that the older svs12-plus had balanced inputs and outputs.
Ok, ymmv of course but what I learned from my whole subwoofer experience is that they were an absolute pain to integrate a single sub. I could always tell where the bass was coming from. Yeah yeah, lower the crossover, center it in the wall, blah blah, total pain. I believe I would have had better results with 2 subs. What I learned is that for the price of 2 nice subs ($1,000-$1300..?), you could spend the extra $ on a full range speaker and be done with it.
Also, when adding a sub, you'll still have to deal with suckout, humps etc. i really think that you'd be happier addressing your room acoustics; it's amazing how much more bass you can get from some bass traps and absorption panels.
If you do still go with a sub, I'd do a sealed design for tighter, more musical bass. I recommend svs. I'd also suggest buying something decent up front, and buy something that you can get another one of later if you decide to go with 2 subs.
Goodluck, and happy listening!