The 21.0 is also a bargain compared to my Velodyne DD18 Plus. Velodyne is currently asking only $100 less than the 21.0's asking price, and while some user interface features are more sophisticated in the DD18 Plus, if I were choosing today I'd pick the 21.0 without hesitating. Those measurements are impressive. In fairness, when I bought the DD18 Plus in 2011 I paid ~$4100 (in a state with no sales tax) with the dealer discount, which would now equate to ~$4900, but I'd still choose the 21.0. For one thing, it weighs over 30lbs less than the big Velodyne, mainly because FA splurged for neodymium magnets and isn't using MDF, which would make it a lot easier to move around to place just right.
Amazing how the subwoofer world has evolved in five years.
Of course, this is all moot. While I'm tempted to move the DD18+ to the HT room, I don't really need it in there, and the damned thing still rocks my new listening room, especially with the mains running full range. I need a 21.0 like I need a new dress. Sometimes I wish I wasn't so practical.