The open baffle servo-feedback bass system used in the GT Audio Works magnetic-planar loudspeaker mentioned by faxer above is the GR Research/Rythmik sub. It is available separately from both GR Research and Rythmik, and can be used with any loudspeaker. It is THE ultimate sub for all planar loudspeakers, as it’s dipole characteristics make it possible to blend seamlessly with them as no monopole woofer can (even in swarm fashion). It also features the Rythmik servo-feedback system, which when combined with the virtues of ob bass, creates the best bass reproduction you’re ever likely to hear. It has found favour with owners of Maggies, Quads, Eminent Technology LFT’s, Acoustats, and of course GR Research speaker owners.
But the Magnepan "Project" loudspeaker is a complete package, including the "midbass coupler" used in the 30.7, as well as the fantastic Maggie ribbon tweeter used in the 3.7i/20.71/30.7. It will be interesting to compare the Magnepan OB woofer system with that of GR Research/Rythmik. The GRR/R uses 2 or 3 (or more!) 12" woofers per side, the Magnepan eight 6.5" per. The GRR/R uses servo-feedback woofers, the Magnepan DSP. Should be interesting!