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.
