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.