I built my x-statiks about a year ago. Every time I sit down and listen, I am amazed at their preformance. Makes me smile. Now I am thinking about some GR subs to go with them.
I purchased my X-Statik's over 12 years ago, and I'm still amazed at their performance.
Definitely build a pair of OB subs if you can. You won't regret it. I would love to have a pair of OB subs again, but again, I fear my room is simply too small for a pair of OB subs AND a pair of speakers along the front wall. It would end up being too cramped and congested I think. This is why I will most likely be going with a pair of the Rythmik F12G subs instead.
a.k.a. - Not much bigger than the Polk subs I'm using right now, which do a surprisingly good job actually. They just sort of give up the ghost by 25 Hz or so. And they do tend to give up some detail at times. The 10+ years that I've owned them though, they've never given me an issue. That's gotta say something for them.