Salk builds into some of his loudspeakers a Rythmik servo-feedback subwoofer, which automatically raises tremendously the speaker's capabilities. Very few even "full-range" speakers are truly capable of reproducing at lifesize SPL the 20-40Hz octave. Incorporating a Rythmik sub into his speaker not only makes that possible, but taking the bass out of the Salk woofer also improves that driver's performance.
Jim Salk also builds incredible subwoofer enclosures into which he installs a Rythmik 12" or 15" sub. His sub enclosure is the best designed and constructed I've ever seen! He uses "honeycomb"-style bracing, which you can see on the Salk website. He also increases the enclosure's internal volume from Rythmik's own 3cu.ft. to 4cu.ft., for increased efficiency and output capability.