My first sub was a DIY servo design ca. 1980-something. I built it to go with my Maggies because other subs at the time were too pricey for a college kid and I was not happen with their performance, mainly their transient response. Servo improved step response, let me flatten the frequency response, and compensated several distortion mechanisms. It outperformed most any sub I had access to at the time (working for a dealer -- as a tech -- and involved with several others).
Fast forward about thirty years and I needed a new sub. After researching I found Dr. Brian Ding's Rythmik subs used a very similar servo design and the guy is very sharp (we have similar day jobs). I ended up with four sealed F12's in my little room -- not for output, but to help smooth the response. I run them as a mono group; I went back and forth on stereo subs many times over the years and always found stereo was (much) more trouble than it was worth (other folk differ). I like sealed because they had better transient response than ported (largely alleviated with either a servo, good design and large amp, or both), don't have to worry about "chuffing", and I get a little lower in-room response since they roll off slower than ported designs (not usually a big deal now when e.g. Rythmik's 3 dB corner is ~14 Hz; back then, 30~40 Hz was common for a sub, so the extra octave or so a sealed sub provided was a much bigger deal).
So, I really like Rythmik, but am very biased and there are a number of good choices these days.
Even in a large room, or maybe especially, I'd go with two subs even if a little smaller than one large one simply to better smooth the in-room response. One someplace up front and the other near-field or as close as possible, or otherwise placed to help compensate any SBIR/room mode nulls at the listening position (see the Welti or Geddes papers).
FWIWFM - Don