All right, I was gonna leave the details of woofer "speed" to those who cared enough to watch Danny Richie’s Tech Talk 6/Fast Bass YouTube video, but in light of the akg_ca post directly above, I’ll say more.
The Vandersteen is indeed a very fine sub, one with which I am quite familiar (Brooks Berdan was one of Richard’s biggest dealers). But to say "The mass of a larger driver will not allow it to respond as quickly as the Vandersteen 8" drivers" is to over-simplify what is actually going on. Again, for a very detailed explanation, watch the video.
At one point in time, Rythmik Audio was offering sub models using 8", 12", and 15" drivers (he has discontinued the 8", and added an 18"). Contrary to common audiophile myth, an 8" woofer is not necessarily "faster" than a 15", 18", or even 24". I have an 8" sealed woofer, and my open baffle 12" and sealed 15" Rythmik subs sounds "faster" than the 8".
To understand why I used quotation marks above, I implore you: watch the video! As a simplified teaser, I’ll just say that the perceived "speed" of a (sub)woofer is not a matter of how fast a driver will respond to the signal (bass frequencies are quite slow), but in how fast it returns to rest when the signal stops. Yes, the moving mass of a driver (the cone) and its stored energy (watch the video ;) is part of that phenomenon, but so is the motor (magnet structure), it’s suspension, the cone material, and numerous other factors.
But imagine a driver which includes not just a voice coil, but a sensing coil which electronically applies the "brakes" when the signal ends, forcing the cone back to it’s rest position---faster than an uncontrolled cone, whatever it’s cone size. That’s servo-feedback, a patented form of which Brian Ding employs in his Rythmik Audio subs. Perhaps you have heard servo-feedback woofers before. I had: I owned a pair of Infinity RS-1b’s for a time, which just happened to use multiple 8" servo-feedback woofers in it’s bass towers. Not even close to the sealed-enclosure 15" Rythmik.
Now, imagine a servo-controlled driver installed not in a sealed or ported enclosure, but in an open baffle frame. Harry Pearson loved the sound of the twin bass panels of the Magneplanar Tympani T-IVa dipole planars (a pair of which I own), using them as woofers in his Tympani/IRS super speaker. I too love that kind of low frequency reproduction, which replicates the sound of upright bass, cello, the lower registers of piano, and of course drumheads like no other woofer I’ve heard. Until I built a pair of GR Research/Rythmik Audio Servo-Feedback OB/Dipole Subwoofers. THE sub for planar loudspeakers!