I do like that Danny's Sub being able to crossover up to 300 Hz.
Of coarse playing up that high makes them no longer just subs. It is then covering the mid-bass and lower vocal harmonics as well. I like to keep them at or under the 200Hz range just to keep from splitting a critical vocal range up into several different drivers.
Playing subs up to that range also requires that they be next to or part of the main speakers as distance from them gets real critical regarding integration.
As for what approach to use can come down to room size as much as anything.
I wouldn't want to play the subs in a swarm configuration up real high either. Keep in mind that as frequency increases the directionality also increases. If the main speakers can play down into the 40 ro 45hz range pretty solidly then integrating them to a sub gets much easier.
I LOVE the open baffle servo subs. They do EVERYTHING well. But they also have to be placed well out into the room. I have my Super-V's a good 5 feet or more out into the room. Some rooms just won't allow that, and the Swarm is a good alternative.
What does it mean when said it flattens all room modes? Looks like it heightens all of them.
Think of it as dropping four golf balls in the water about 16 feet apart verses all of them in one spot. All spread out evens out the waves and some even cancel each other out. All in one spot makes one big wave. Now imagine putting a box around all of them.
Plus you can run the back subs out of phase from the front subs and really even out the way they load the room. By moving them around a little you can actually get them to cancel out each others output to some degree in areas where you might have a peak in the room.
Another thing to keep in mind is that if you can use multiple subs verses a single monster, the amount of movement needed to create the same SPL levels are much less for a group verses a single. So what you get is much less moving mass (typically) and MUCH less stored energy. The subs return to rest more quickly.
Plus those long exertions can cause flexing of the cone and a real increase in distortion. So the multiples REALLY have an advantage. Now throw the additional control of a servo system on top of that and you'll hear resolutions levels in low frequency ranges that you didn't realize ware there.