How do SVS subwoofers compare to REL?

@icanskate - which model PSB sub did you select for your Magnepan 1.7i speakers?

My experience with some of the equipment you are considering is a bit tangential at best -- I owned Magnepan 1.6 QR speakers for several years, but never tried to use them with subwoofers. I spent some time at friends home who used REL subwoofers very successfully with Spendor SP100 speakers. I currently use a pair of SVS SB16 Ultra subwoofers with KEF 207/2 speakers.

I think the SVS are a great value and don't give away anything to much more expensive subwoofers since the introduction of their DSP amplifiers, newly designed 16" and 13" drivers, and integration with a very intuitive app. The SB and PB4000 share the same DSP and app control with the SB and PB16 Ultras -- three parametric eq bands, three presets that can be customized by the user, real time gain adjustment. The only thing they won't do is play a test signal and auto-calibrate themselves, but with the flexibility and control of the app, tuning the SVS subs from the sweet spot without having to get up to make adjustments is a piece of cake.

I'm very pleased with the performance of the SVS subwoofers and how seamlessly they integrate with the KEFs.

I use a single REL T5i in a 12' x 20' room with Harbeth P3esr small monitor speakers.  The specifications indicate the Harbeths start to roll off at 75 hz.  Whatever the specs, the influence of the room will dominate the perception of bass.
I have somewhat limited options for monitor placement.  For optimal listening with my system in my room I pull out the speakers into the room and have more of a near-field listening experience.  Bass frequencies being much less directional (perceptually) allows me to move my sub around to get the most enjoyable results.  When I was setting this system up, I was on my hands and knees scooting the sub around until I liked what I heard.  It's not a symmetrical arrangement, but it allows the little T5i to adequately (at least for my taste) activate the room.  I may try DSP room correction some day, but I find that jiggling things around and listening works pretty good.  I would work with placement and room corrections before surrendering to the digital dark forces.
Keep in mind this works with my system in my room for my listening tastes.  I don't listen very loud, and I prefer smooth and buttery sound over bright and articulate.  My system sets up a somewhat miniaturized 
user experience, but it's all there – detail, clarity, soundstage, dynamics, and, most importantly, an intangible I'll just call musicality.

One more thing- in talking with several so-called experts calling around before buying I was told by all that 10” or smaller is ideal for tight bass that is fast/quick and better suited for musical enjoyment.  You want the subs to keep up with your speakers in reproducing the music.  Larger subs go lower and match better for home theater use.  I have a larger room than yours and two REL T9/I subs did the trick beautifully.