As shown above, the big Accugroove appears to use the Beyma 21L50 driver. I've not seen confirmation, but this is the only 21" driver that comes close. It has a free air resonance of 33 Hz, so it isn't gonna do much below that, hype or no hype. I do not believe the claimed sensitivity of 98 SPL with -6dB at 19 Hz. Playing a video of the Tacoma Narrows Bridge collapse in lieu of testing results is just hype. Show me the measurements.
I am puzzled by the 4.8 cabinet volume. My records show an external volume of 8.43 cubic feet, so I would think the net volume would be larger. If so, Accugroove could possibly be using the extended bass shelf (EBS) tuning to extend response down lower at the expense of transient response.