The Rythmik F12 is great, and is also available with a paper cone instead of aluminum, model F12G (for GR Research.). For those wanting to move more air, there are also the F15 and F18, aluminum cone only.
For those with dipole loudspeakers, there is the amazing OB/Dipole Sub, a collaboration between Rythmik’s Brian Ding and Danny Richie of GR Research. Available as a kit only.