As another indicator of the sound characteristic of the Rythmik F12G sub, one listener's reaction upon hearing it was that it sounded "leaner" than other subs. To me that's a compliment!
Hartf36 mentioned the Rythmik's adjustability. Brian Ding considers the phase matching of a sub with it's partnering loudspeaker a major priority, and his phase control is the best I've seen. It is a continuously variable rotary knob that allows the close alignment in time of the sub's woofer with the speaker. The sub's position in the room can be selected with the matter of the room's modes (the locations in the room where low frequencies resonate the most---a function of the room's dimensions, causing boom and overhang) as the priority, the phase between the sub and speaker then adjusted via the sub's phase control. Most sub's force one to choose between the two.
When a sub is not precisely aligned with it's speaker, not only is the sound quality of the pairing reduced, so is it's output. With the sub and the speaker slightly out of phase at certain frequencies, their opposing polarities cause a cancellation of combined output at those frequencies. The Rythmik phase control allows the minimization of that phenomenon.
A fair number of the contributors on Rythmik's AVS Forum are Magneplanar speaker owners; the subs are finding favor with that and other planar speaker owners.