Best sub match for Magnepan 3.5's

I am looking for the best match for my Magnepan 3.5's, size and price are not an issue, but I am looking for something powered and will go low. Right now, I am running a Conrad Johnson ART 15 preamp, Conrad Johnson MF2200 amp, and I am going to go with Bryston's 10B active crossover. Any experience or suggestions would be appreciated. Thanks.
the best that work for me is the rel sub.
I have Maggie 3.5s and a Velodyne ULD 15II that works quite well. I will say I like Rock n Roll and one day I may purchase a second to match each Maggie that will be awesome. I like my bass so I can tell the sub is behind the left speaker (center placement not an option). I have not heard a Rel sub but for me I would rather have 2 mint condition Vel ULD 15II then one Rel sub.

Many believe optional stands really ad to the bass. Thats my next upgrade.
I also have 3.5s with a ULD15-II. This is truly a great combination. I have tried as augment and with the crossover and find that using the crossover takes a huge strain off the bass panel freeing up a lot of the muddiness at high levels. All in all a great system, so much so that I am looking for a matching 15-II.
I guess I'm an oddball....but I used to own a pair of Timpanys and after reading an article...I positioned them about 6, from the back walls and as close as possible to the sidewalls, edgewise. I used an Acoustat TNT 200 and an Audio research SP-14 tube pre and I did'nt need a sub...the bass impact and delineation was phenomenal!. The article said that planars will cancel the bass when placed too close to rearwalls, also the sidewall proximity helps quite a bit, when done properly. Hope this helps you.