Alright, I have a Def Tec 15" sub and am not thrilled, I know the ones listed are better, but Velodyne is simply the best sub that I have ever heard/felt in my life. In the past 4 years of high-end home A/V work the Velodyne HGS/DD series have blown away anything I have heard before.
Now, I just lucked into a great price on a HGS-18 / coffee table and think that it is time to sell my old (immaculate) PF15TL+ Def Tec and I am strongly questioning whether I have any reason in the world to keep my Velodyne HGS-10.
The word is that the DD-15 is superior to the HGS-15, which, as hard as it is to imagine, is reason enough to believe that you will be overwhelmed with smooth clean bass for a long, long time of hapiness.