@johnnyaa, the statement above that REL subs connect "unlike anything else" is incorrect. Rythmik subs also provide for "High level" connection (from the main power amp, on binding posts), but unlike REL's offer the alternative "Low level" (from a pre-amp, on RCA jacks) capability, allowing one to try the sub both ways.
The Rythmiks also sound excellent, with 12", 15" and 18" models available. Quite a few Maggie owners have posted on the Planar Speaker Asylum about the mating of Rythmiks with their planars. A VERY special sub option is the remarkable Rythmik/GR Research OB/Dipole Sub, which blends with planars like no other sub available. Read all about it on the GR Research AudioCircle Forum.