REL has a great reputation amongst advanced audiophiles, but they can't be considered in the same category as the OB/Dipole Sub. REL's, the Rythmik F12G, and other excellent sealed box subs are a completely different animal than the GR Research/Rythmik Open Baffle/Dipole Sub. Completely different in design, sound, and acoustic behavior, in the same way a Dipole speaker is completely different from a box speaker. Their dipole operation and figure-of-8 radiation pattern, where they need to be placed in the room, how they integrate with a loudspeaker, their maximum output capability, their lack of a resonant box, their characteristic sound--- "leaner", cleaner, etc. all different from any and all box subs, whether sealed or ported.
I liken the difference between box subs and the OB/Dipole as the difference between an acoustic/stand-up bass and an electric. The electric is rounder, fuller, fatter, the acoustic quicker, more snap and attack, less overhang and plumpness. The OB/Dipole is THE sub for ESL's, Magnetic-Planars, Ribbons, and other highly transparent loudspeakers.