I use 2 Velodyne SPLR 8s - less than 12" cube. They're quite good, but from my research it seems that small subs always trade some performance for size at any given price point. Thus, the SVS subs at $600 per offer much better performance on overall key specs (extention, distortion AND group delay) than any small sub at any price. Now, can you hear the difference? That one I can't help you with.
Marty
PS - the little REL subs test well for group delay ("fast") but pretty horribly on extention and distortion.