Rel subs with Magnepan 3.6's

What Rel subs (I plan on buying 2) on a budget work well for 2 channel using Magnepan 3.6's? My room is roughly 14x35 with my system on the long wall. The ONLY other sub I am considering is SVS SB2000. However strongly leaning towards Rel as they hook up to the amps, so better synergy. Thanks for any input!
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!
The Rythmik plate amp has "High Level Inputs", allowing you to take the signal for any of the Rythmik subs (sealed, ported, and the OB/Dipole model) from the output terminals on your Quicksilver (and most other) power amps, just as the Rels do. That is the PEQ3 version of the Rythmik amp, which also provides single-ended RCA connections; also available are two different XLR versions, for anyone desiring balanced connection to a pre-amp. Available in 370, 600, and dual channel 400/400 watt versions. That doesn’t sound like a whole lot of power, but the Rythmik drivers are unusually sensitive and efficient, a result of their very low mass and Servo-Feedback design and control.
Good to know, so which Rythmik sub or subs (2 would be best?) with Maggie 3.6's in a room roughly 14x35 with 8 ft ceilings? Just curious what you would recommend. Thanks so much for pointing me in this direction!
REL mates very nicely w/ Maggie speakers. If you can get a demo prior to any purchase.

There are a few Maggie owners using either the F12 or F15HP subs. The F12 has, as you would suspect, a 12" woofer, the F15HP a 15". They sound identical, but the 15, having a bigger cone and stronger motor (magnet), as well as bigger sealed enclosure, higher power (600w vs 370), and greater driver excursion, has more output. The difference in price is about $200, I believe. Both subs are sealed, and very clean, excellent for use with planars. The Rythmik plate amp has extensive controls, the most important being a continuously-variable phase control, an absolute requirement for a blendless seam between speaker and sub. Do NOT buy a sub without it! Rythmik also offers ported subs, which are great for extra output at and above port tuning, for large rooms such as yours (35’ length!). There is also a sealed dual 15" model with two drivers and a dual channel 400/400w amp. It’s around $1500, last I looked. In development is an 18" model, slated for release later this year.

The thing is, all Rythmiks are Servo-Feedback designs, the operating parameters of the woofers tightly controlled by the very sophisticated Rythmik feedback system. For absolute best sound with planars, the way to go is the OB/Dipole sub, as described above as well as in a new thread on Audiogon today ("Need advice on adding subs") in which I describe in greater detail the sub’s design. The OB/Dipole sub is as different from boxed subs as planar speakers are from boxed speakers. Think about it: The sub has the same figure-of-8 radiation characteristics as a planar speaker, with nulls to each side. It has no resonant sealed or ported box, same as a planar. It excites fewer room modes, just like a planar. Being an Open Baffle/Dipole sub, with the Rythmik Direct Servo-Feedback controlling of the drivers? It’s a game changer---the absolute State-Of-The-Art in subwoofers!

But the OB/Dipole requires some effort, as it is a DIY kit, and runs about $2000/pair. I’ve never heard an REL, but it must be real good, as a lot of people here like it. There are also Vandersteen sub fans, and they’re good too. I’ve heard them (at Brooks Berdan’s shop), but they are no competition for Rythmiks!

Another vote for the Rythmik Audio F15HP. Get 2. Build quality on these is second to none. I’m using one with my 3.6s as well some subwoofer towers.

I like the idea of the open baffle/ dipole except for the extra build work involved. Despite the theory, I’m not sure given the large wavelengths at sub frequencies OB/dipole subs would make that much difference in room modes. You could use minidsp /REW to flatten the response and equalize the subs (only) at the listening position and possibly end up with a better result.

Oh, You’ll have useable response to below 20 Hz with the F15HP.