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.
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.