The OB/Dipole woofer system seen in the GT Audioworks are made by GR Research/Rythmik, and feature the Rythmik servo-feedback operation. They are available separately by both those companies (as a DIY kit only), and can be added to any loudspeaker, just like a "normal" (non-OB/Dipole) sub.
The basic kit is a pair of woofers and a plate amp, the latter including a "shelving circuit" to counter-act the dipole cancellation inherent in all OB speakers and subs. Both 8" and 12" woofers are offered, the base version using a pair of drivers in an H-frame. But you can add as many woofers (and associated plate amp) as you want, the GTA speaker having four woofers per side. THE sub for planar loudspeakers!