Best sub match for Magnepan 3.5's

I am looking for the best sub match for my 3.5's. Size and price are not a concern, but I am looking for something powered and that can go low. I currently have a Conrad Johnson ART 15 preamp, a Conrad Johnson MF2200 amp, and I am going to use a Bryston 10B active crossover. Any suggestions would be helpful. Thanks.
I cannot comment on the Magnepan 3.5's and a Sub, I can only reply that in my search for a musicial sub I felt the Martin Logan Depth and the Rel Strata lll were 2 of the most musicial sub's I had heard. Both very fast, tight, and a lot of authority behind them.
That is my 2 cents worth.
I have the ACI Titan II LE that mates well with my 3.6, some say the Vandersteen 2qw and REL Strata III are also a good matches. Check out the new Home Theater magazine for a unique test and review of some subs.
Try asking this over at MUG a lot of folks are running subs and they have lots of experience with it, you may be able to search this topic and find a lot of information quickly. I believe a lot of them are using Rel subs and love the results, but check and see if there's a new favorite- I don't keep up to date with subs.
I am running a pair of Vandersteen 2wq's with my 3.6's. The sound is satisfactory. I think the bottom lacks a touch of definition that one can hear on large full range speakers such as the Soundlab electrostatics but, this could be true with other subs as well when mated to the Maggie 3.6's. I have not tried others so can not make comparative comments. But I am thinking about an REL......
I'm using a Rel Strata III with Magnepan 1.6QR's.
Excellent match, easy to configure.