The best subs I had when I owned CLS were Celestion System 6000 dipole subs. Dipole bass is wonderful, and will naturally match the dipole nature of your speakers.
However, there are no current makers of dipole subs to the best of my knowledge. Fortunately, it is very easy to build one, much easier than traditional subs.
Here is a link that explains how:
http://www.linkwitzlab.com/proto.htm#PW1
I've also owned Entec subs and they are very good, but for a dipole speaker especially, I would go with a dipole sub.
However, there are no current makers of dipole subs to the best of my knowledge. Fortunately, it is very easy to build one, much easier than traditional subs.
Here is a link that explains how:
http://www.linkwitzlab.com/proto.htm#PW1
I've also owned Entec subs and they are very good, but for a dipole speaker especially, I would go with a dipole sub.