I'm been looking to get a "fast" sub to go with, not just my Sierra-1, but other high efficiency speakers (fostex breed), and it seems the Rythmik with the GR drivers would be perfect.

http://www.rythmikaudio.com/GR_drivers.html

"The GR drivers use a lower mass paper cone, which has a slightly better transient response. The result is slightly better dynamics. At higher output, due to less cone stiffness, the sound may become softer due to the pressure placed on the cone. "

I'm all for better transiency, if not enough output I can always get 2.

Anyone have experience with an F12 (or a pair!) with GR driver in a 2ch music env?