How do SVS subwoofers compare to REL?

With subwoofers the room interaction is far more important than the quality of the actual sub (within reason, of course). In this room the Schroeder frequency is about 150 Hz, and below that you will suffer room modes, i.e. resonant frequencies at the room’s specific dimensions. These produce pretty horrible peaks and dips in the response and are audible as boomy one note bass.
You are lucky with Magnepans as these are dipole speakers and dipoles do not excite room modes nearly as much. That also makes for a pretty big contrast with any normal subwoofer. If you have room for them, I would therefore suggest to investigate the dipole subs by Rythmik. Unfortunately they only do a kit, but there are subcontractors who will make you a cabinet (and to your taste).
Beyond that, two (or ven four) small subs will always beat a single big one with respect to smoothness. So, for a given budget, two small subs will give a smoother and more accurate response, but a single big one will be better at HT explosions. See here for an introduction: https://www.google.nl/search?q=welti+geddes+multiple+subs&ie=utf-8&oe=utf-8&client=firef...
Another interesting though bulkier option is the Swarm System by Audiokinesis, using four subs.
In every case I would add an Antimode 8033 room equalization unit for much improved bas quality at a modest price. These connect the sub at line level, however. If you only have speaker level connections you will need an attenuation cable, but that is no big deal (make sure to get a hefty one given your big power amplifier).
Both REL and SVS are respected subwoofer manufacturers. REL are nicer looking and SVS are usually better value for money (and offer a very good money back guarantee). These days REL is no longer the only one to provide for speaker level connections (though not all SVS models allow for this). In any case, if you would be using the AM 8033 this is irrelevant. In my experience REL are dead right, however, that subwoofers should only come in at the very lowest frequencies.
As for sub size vs room size, I would avoid large subs in this medium size room. For music, even two SB1000 subs will give you quite a bit of extra bass, let alone four. HT is a different story.
In my own case, I bought a B&W PV1d sub to combine with my Quad electrostats (if only for its clean and modern looks, but also for its higly adjustable dsp crossover). This only integrated properly once I added an Antimode 8033. Next on the upgrade list is a second PV1d. Not really value fo rmoney, I am afraid, but my wife likes the looks, and so do I.

True, that was also the case with my B&W PV1d subwoofer and my Quad electrostats - until I installed an Antimode 8033 Room eq unit. I honestly think that the more you do to solve the room modes issue the better.

I really think ’speed’ is not a property of the subwoofer driver, but of the room acoustics. If you look at waterfall graphs of low frequency output, you can see that the lingering of some low fequencies is at room modes. Deal with those, and your ’slow’ subwoofer suddenly becomes ’fast’ (been there, done it). I suspect that part of the explanation for the observation that big woofers are slower may therefore be that they potentially generate more room modes.
As for subwoofer design, I guess that everything else being equal sealed enclosures are more beneficial for music than vented ones, with designs with passive radiators somewhere in between. Vented ones are often preferred for HT and massive explosions (if that kind of movies is what thrills you) because they can go deeper and louder. SVS offer both designs. The Audioholics forum has many detailed discussions and measurement data.

It is true that under those conditions you have excluded a lot by defining it away, but in reality big drivers are likely to go deeper. That was my point. However, I am not excluding the possibility that big drivers are flabby and not as accurate. They would certainly need more control from a powerful amplifier. My own subwoofer (B&W PV1d) has two opposing 8 inch drivers, and is reputed to be 'fast'. Yet, without the room equalization that I later added this sub did not seem 'fast'. Originally it was certainly woolly and boomy and a problematic match for my ultraclean electrostats.

Indeed and I think we effectly agree. I agree that there is little or no useful music signal below 25 Hz, and to be sure, for this and other reasons I am not a fan of huge woofers either. What I tried to say is that the issue of setting subwoofers up properly is largely to deal with the room modes.

As for DSP in subwoofers, it is used for two things and it is important to distinguish them.
1 As a means to shape the frequency response (boost the lowest frequencies), and as a way to manage the demands put on amplifier and drive unit. Mostly, this is done by reducing the deepest bass output at higher volume, to limit distortion and potential damage to the driver. DSP can also be used as a more refined digital crossover.
2 To measure and then correct in-room response. For this, the system has to measure in-room response with a test tone sweep and a calibrated microphone, construct a correction curve, and then apply that curve.

These days, many subwoofers do 1) and some also 2), such as the bigger models by Velodyne and B&W, but rarely as well as the Antimode 8033.

I had a quick look at the SB16 specifications. From what I can see it can do some basic manual equalization, but it does not measure response, let alone correct it automatically - but correct me if I am wrong. Beyond that, I think is is an enviable subwoofer for people with really large rooms.