That Velodyne DD-12 is one hell of a sub. It is sealed, has built in room correction, has a servo feedback mechanism controlling woofer excursion (this is not something you will find on lesser subs), uses a digital amp, so doesn't generate much heat. Plus, the Velodyne DD series has garnered outstanding professional and user reviews. I would give that Velodyne some serious consideration.
In fact, if money is not an issue, I wouldn't hesitate getting two of the Velodynes. There is something to be said about having stereo bass...