how does the MFW-15 compare to the SW-12B? What's a better match for a 2150 cu ft room (see floor plans here) - one MFW-15 or a pair of SW-12B in parallel? What will match better with N3/N3S fronts?
I think either of them can be made to sound outstanding or horrible. The box construction and primarily how solid and non-resonant it is makes a huge difference in how it sounds.
When it comes to sound quality another important aspect is how fast it can stop. And everything that you do to make it play really low fights against that aspect. To play low you need a bigger diaphragm, heavier moving mass and low Fs. To stop really fast you want a lighter weight moving mass and you try to limit the exertion to shorter strokes. That gives you less stored energy.
So weight your objectives and go in a direction that will best get you there.
For music only and quality in mind then I might go with four SW-12B's in a sealed box with a SA-1 amp or something with a transform circuit.
For home theater I might go with one (or maybe two of them) MFW-15 in a ported box and a big amp on it.