Rel subs with Magnepan 3.6's

The budget Rel's I looked at have a passive radiator in addition to an active, a bad idea with speakers as good as yours. The SVS SB2000 is just another average budget sub---a woofer and an amp in a box. For the same $ you can get a Rythmik Servo-Feedback sub, much higher in performance than either, used by several Maggie owners I know. Sterling Sound (the renown mastering facility) has Rythmiks in all their monitor systems. If you are curious as to why a passive radiator in a sub is a bad idea, Rythmik design engineer Brian Ding covers that subject in the technical papers on the Rythmik website.

The svs is for movies.   Sealed sub for magnepans only.  Nothing ported  or with a radiator is gonna keep up. Really in magnepans if you want better bass it's better to jump up to the bigger panel  in this case the 20 series.    What amp / preamp are you using.   The Maggie's will show the weakest link 

I'm mostly with you timber. There is one very special sub that is THE sub for use with planars, but at about $2000 a pair, and a DIY kit at that, I didn't think sonicsoul would be interested. It is the OB/Dipole Sub offered by GR Research in conjunction with Rythmik Audio. Two 12" woofers in an OB/Dipole H-Frame (not a sealed box, not a ported one either, but two woofers optimized for free air use), powered in a closed-loop feedback circuit by a Rythmik plate amp incorporating a shelving circuit to compensate for the acoustic cancellation endemic to OB/Dipole designs. The only Open Baffle Dipole subwoofer with Servo-Feedback controlled woofers in the world, and unusually suited for augmenting planar speakers, whether magnetic-planar, ESL, or ribbon. The sub blends seamlessly with even the most transparent, high resolution speakers available (I use them with Quad 57's, Magneplanar Tympani's, and Eminent Technology LFT-8b's), becoming undetectable as a separate entity. Details available on the websites of the two companies. Co-designed by Danny Richie and Brian Ding, and brilliant!

Thanks for the recommendations. Rythmik is actually one other I have heard is a good match. I use Quicksilver V4's with a high output pre and really enjoy it. I was leaning to Rel only because it played from the amps signal and not from an output from the pre. I will look into it, but you have helped point me in the right direction, or avoid buying a HT sub or one that does not match well...thanks!