Building Resonance Free Subwoofers

@bdp24 Exactly! If you want to get anywhere near a live performance a powerful subwoofer system is mandatory. I might add that approaching realistic levels requires more gain than one would expect. Standard speakers, even large ones can not handle this. Room control, digital EQ and crossovers are important additions. I would even go so far as to say they are mandatory for the best performance. They also make integration a breeze. 

My last set of commercial subwoofers were early Velodynes and like yours the foam surround disintegrated in 5 years. Garbage. That is when I started building my own. As you note there are many roads to Rome. Kits are a wonderful and inexpensive way to build a subwoofer system. Dayton also offers subwoofer kits and they perform just as well as most commercial units even if they are not as sophisticated in the finish department.  Box enclosures do require extreme bracing. You should see how Magico builds their Q series subwoofers, wild. They are also balanced force like mine. KEF and Martin Logan also make balanced force subwoofers, but continue to use box enclosures. Aside of cylindrical enclosures I would also suggest that like Audio Kinesis, they make their electronics packages outboard. They also need to add high pass filters. 

@mijostyn IIRC the Dayton Audio amp that Duke LeJeune recommends for his swarm has a high pass filter but it's fixed at 12 dB per octave. So not very flexible. It's why I decided to use my own active crossover for the swarm I put together.

When I spoke with Roger Modjeski (of Music Reference) about mating subs with the old QUAD ESL loudspeaker---of which he was a huge fan and owner, he recommended employing a crossover frequency of 100Hz, with 4th-order filters in both directions (high pass and low). He maintained that the bass panels of the ESL had a rather pronounced resonance in the 50Hz-100Hz frequency band, and benefited from not allowing the panels to reproduce those frequencies.

The subject of whether or not to use a high pass filter with the main speakers is a matter of some disagreement. Making a seamless transition from speakers to subs is not easy, but having as good a set of subs as possible is of course the place to start, whatever crossover filter characteristics one prefers.

It is my opinion that relieving the loudspeakers the duty of reproducing very low frequencies can greatly benefit the loudspeaker’s reproduction of the higher frequencies the woofer must also reproduce (commonly into the midrange). Employing a crossover frequency of 100Hz with 4th-order filters is a good general recommendation. Finding the best room locations is the next order of business.

Place them where they best address the room’s high and low pressure zones, where bass frequencies either disappear into black holes or "ring" far past the point where the signal has ended, the result of the room’s dimensions creating those zones, referred to as "eigenmodes". Tho location of the "modes" of any given room can be found by entering the room’s dimensions into one of the mode calculators findable via a Google search. If at all possible, do NOT place your subs in those locations.

@clio09 The swarm is built to a price, but I agree crossover flexibility is an important factor when is comes to integration. The most flexible by far are digital.

@bdp24 You won't get an argument from me. Roger Modjeski was absolutely right. Big flat panels have low frequency resonance issues. Sound Labs deals with it by using, I think it is 10 different sizes of panel. Acoustat used felt pads on the back side of the panels to dampen them. The best solution is to cross out at 100 Hz, but you have to use a very steep curve or you wind up with subwoofer in your midrange which is poison. I use a 10th order filter, 48 dB/oct. The test is listening for voice in the subwoofer system, there should not be any or you smear the image. With shallow filters, even 18 dB/oct, you have to cross out below 60 Hz to keep the sub out of the midrange. Another advantage of having your subwoofer system active from below 100 Hz is increasing the gain on this segment between 10 and 100 Hz allows you to get the feel of a live performance without having to resort to ear damaging volume levels. 

With multiple subwoofers placement is not as big an issue with point source systems. Having a line source system I have to arrange them to form a linear array or they will fall behind the main speakers. Another benefit is the bass response throughout the room is very even with a slight increase at the boundaries. 

Most impressive in all respects. I just finished my bass system consisting of 2 Magnepan DWM's for the lower 3 octaves and an isobaric sub, which I thought was heroically braced until I saw your design. Nicely done!