My short answer is, two smaller subs instead of one bigger sub.
The following is my long answer.
Off and on over the course of a decade or so I tried building a sub that was "fast" enough to mate well with Maggies and Quads, on the theory that there might be a market for such. I built sealed boxes, low-tuned vented boxes, transmission lines (many different geometries), equalized dipoles, aperiodics, isobarics, and pretty much anything that seemed promising except for a full-sized horn. Some were better than others, but none passed the test.
The one day a really smart guy, Dr. Earl Geddes, taught me that the problem is room interaction, and regardless of how "tight" and "fast" a sub is, the room will impose large peaks and dips that will dominate its response. It is the peaks that are especially detrimental, in that they decay slower than the rest of the spectrum. His suggestion was to use four small subs asymmetrically distributed, such that each produces a different room-interaction peak-and-dip pattern, and the sum of the four dissimilar peak-and-dip patterns would be much smoother (and therefore much "faster") than any one alone.
This made sense to me. I was aware of an AES paper that showed a dipole has significantly smoother in-room bass than a monopole, and a dipole is two monopoles back-to-back with the polarity of one reversed, plus a path-length-induced time delay between them.
The general principle I learned from Earl is, the more intelligently-distributed bass sources within a room, the smoother the in-room bass. Two subs are potentially twice as smooth as one, and four subs are potentially twice as smooth as two. A dipolehas roughly twice as smooth in-room as a monopole in the bass region, so four intelligently-distributed monopole subs are theoretically comparable to two dipoles.
If you do a casual survey of relevant posts by Maggie and Quad owners, I think this is what you will find: Those who have tried a single sub usually go back to using no sub, and those who have tried two subs usually keep them in the system. I think this is because two subs exhibit less in-room smoothness discrepancy relative to a pair of dipole mains than does a single sub. So don’t fall into the trap of thinking "I’ll try one sub and if it’s an improvement then I’ll add another." One sub probably won’t be a worthwhile net improvement.
So to get back to your question, I believe two intelligently-positioned subs would be smoother (and therefore potentially "tighter") in-room than just one. Some EQ or other adjustability might be called for, because the amount of boundary reinforcement varies significantly from one room to another. If the two subs have continuously-variable phase controls, that might be sufficient adjustability: Set their phases 90 degrees apart to begin with, and adjust their relative phases from there (along with your adjustments of level and frequency).
I don’t mean to dismiss the qualitative differences between different subwoofer models, but I think the room-interaction advantage of two small subs intelligently distributed would probably outweigh the benefits that the single larger (more expensive) sub has to offer.
Duke
distributed multi-sub advocate
