Apologies in advance to readers of Robert Green's Yahoo group... much of this is re-cycled...
Often discussions of "which sub for Maggies or Quads" includes recommendations of dipole subs, and for good reason: Dipoles blend well with dipoles! Let's take a look at WHY dipoles work well, because I think there's a common misconception. And let's look at multiple monopole subs, and see what they do better and what they do worse, and see if there might be a "best of both worlds" approach (spoiler alert: I think there may be two "best of both worlds" approaches).
Okay let's start by dividing the bass region into two zones (and I realize these aren't new concepts for most of you): The "modal zone", where we get those familiar and unwelcome room-interaction peaks and dips; and the "pressure zone", down below the modal zone, where the wavelengths are so long relative to the room dimensions that all of the first reflections occur within 1/4 wavelength of each other, which tends to put all of the in-room bass energy in-phase. We will come back to these concepts later.
Another concept we want to look at is "de-correlation". This concept is less well-known, but it is the primary acoustic smoothing mechanism for in-room bass in both dipole bass systems and distributed multiple monopole bass systems. We get good de-correlation when the in-room bass energy is adding in semi-random phase. De-correlation is desirable because it results in smooth bass, because no particular frequencies have enough energy to stick out like sore thumbs (or "peaks"). To help you visualize what we mean by the term, good "de-correlation" would be like the foot-falls of a highschool football team running out onto the field. On the other hand, strong "correlation" would be like the foot-falls of the Wehrmacht on parade. The thing to remember is, good de-correlation = smooth in-room bass.
And smooth bass is "fast" bass, literally and perceptually. Because room + sub(s) = a minimum phase system, in-room peak = slower energy decay at that frequency = "slow" bass. So if the perceptual goal is "fast bass", the road thereto is the one marked "smooth bass".
In general, dipole bass really is smoother than monopole bass! In 2002 researcher James M. Kates published a paper in the Journal of the Audio Engineering Society showing that dipoles have smoother in-room bass than monopoles. This wasn't news to the Gradient and Quad and Maggie and SoundLab and Apogee owners out there, but it WAS long-overdue officially-sanctioned peer-reviewed validation.
Now most people who talk about the "speed" (in-room smoothness) of dipole bass attribute it to the figure-8 radiation pattern exciting fewer room modes. But if only few-and-far-between room modes are excited, that's an indication of strong correlation (Wehrmacht)! And we'd expect those few-and-far-between peaks and dips (especially the peaks) to stick out like sore thumbs. So, I think that attributing the in-room smoothness of dipole bass entirely (or even mostly) to the figure-8 radiation pattern is a misconception. Kates' data shows smoother in-room bass for dipoles, which implies good de-correlation, so something else must be going on. Here's what I think is happening:
The primary smoothing mechanism for dipole bass is, the phase relationship between the frontwave and the backwave energy. The figure-8 radiation pattern also plays a role here, so it does matter, but not all by itself. Because of the radiation pattern, the normal-polarity frontwave goes off in one general direction, and the reverse-polarity backwave goes off in the opposite direction. After a few room boundary bounces the twain shall meet again, but their phase relationships are now generally semi-random - which means good de-correlation - highschool football team!
[Not that I'm necessarily the world's biggest fan of highschool football teams, whose members were often the dispensers of wedgies and noogies to us little nerdlings, but at least football gave them someone else to hit and hit hard.]
In general, it takes two intelligently-distributed monopole sources to get the same approximate in-room smoothness of a single dipole source. The energy from the dipole source sums in-room in semi-random phase because the frontwave and backwave start out in opposite polarity and are launched in opposite directions, and the energy from two intelligently-spaced monopole sources also combines in semi-random phase because of their physical distance from one another and dissimilar physical distances from most of the room's boundaries.
So the same mechanism - semi-random phase summation - is at play in smoothing the in-room response of both dipole bass systems and Swarms in the modal region. But the two approaches totally diverge as far as what happens down in the pressure zone! If the energy in the pressure zone is all in-phase (like with four monopole subs), then it sums in-phase. This can result in boom, because in-phase summation is inherently 3 dB louder than the semi-random-phase summation we were getting up in the modal zone. And because of the ear's heightened sensitivity to relative loudness at low frequencies (which is shown in the bunching up of equal-loudness curves south of 100 Hz), a 3 dB emphasis below 40 Hz can sound as big as a 6 dB emphasis up in the midrange region. (This is also an argument for most systems having significant room for improvement in the bass region.)
With a dipole, at very long wavelengths the situation is kinda the same but the outcome is totally different. In the region where all of the reflections are happening within 1/4 wavelength, we have half of the energy with one polarity and half with the opposite! The net sum would be complete cancellation ( making it a "no-pressure zone"?). In practice the cancellation isn't complete, but it is severe, and as a result dipole bass tends to have poor impact unless a) the room is very large (pushing the "no-pressure zone" down very low in frequency) and b) the dipole bass system actually does go very low (which requires a very large baffle and/or a lot of EQ).
So intelligently-distributed monopoles can match the in-room smoothness of dipoles in the modal zone as long as we use enough monopoles. But down in the pressure zone, the monopoles tend towards having about 3 dB of excess energy, while the dipoles tend towards complete cancellation (virtually no bass energy). Neither situation is ideal, and subjectively "too much low bass" can be even more distracting than "not enough low bass".
But, the Swarm has one final trick up its sleeve! We can reverse the polarity of one of the four subs! Now as we move down into the pressure zone, we have 3/4 of the energy in normal polarity and 1/4 of the energy in reverse polarity, so we end up with neither the low-end over-emphasis of the pure monopole (all woofers in-phase) approach, nor the trending-towards-complete-cancellation of the dipole approach. And by reversing the polarity of one sub, we are also improving the de-correlation up in the modal zone. So we end up with smooth bass all the way down, with no over-emphasis anywhere. And remember, smooth bass is "fast" bass.
Okay, is there another approach that can give similar results? I think so, and it's something Siegfried Linkwitz has been doing for decades: Use dipole bass down to the transition between the modal and pressure zones, and use monopole bass down in the pressure zone. Both approaches work well.
Not all rooms will have a significant pressure zone - open-floorplan rooms won't - so not all of the distinctions made above will be equally applicable. The best answers are probably arrived at on a case-by-case basis. But I think that a good distributed multisub system, well set-up, is competitive with a good dipole system across the modal zone and superior to both dipole and pure monopole (all woofers in-phase) down in the pressure zone.