The VTF-2 has been in production for many years now, and is vastly improved today culminating in MKII version, so use caution in comparing. The STF-3, VTF-3, and TN1220HO can easily hold their own with almost any commercial subwoofer out there in their respective price ranges. They all use larger enclosures, 12 inch driver, and more powerful amps than VTF-2. No comparison really as frequency dips below 25Hz, as expected.
The VTF-2 tested in 1997 had 109.7dbs averaged output using 10% distortion limit in output mode, 107.1db's averaged output in extension mode with 91db's at 20Hz. These numbers place this sub is very high company (think near more expensive sub from Paradigm, Velodyne, ..). Numerous advances have been made in the years on this very product. The current STF-2 is $399 (or $349 at CompUSA if you can find one), has more power, more port diameter radial area than VTF-2 tuned for extension. The current VTF-2 is also selling for $449 with free shipping.
The STF-3 is $599, VTF-3 MKII will be selling next month for $699 with free shipping. Amazingly but true.