I used to use the Celestion System 6000 subs with my CLS. They are a dipole open baffle stereo set with active crossover. Great sound, but will not play as loud as some of the monster subs.
Otherwise Kinergetics or Entec subs bought used.
There is a set of Kinergetics at wwww.audiovideologic.com
Otherwise Kinergetics or Entec subs bought used.
There is a set of Kinergetics at wwww.audiovideologic.com