Eh hem!...Subwoofers... What do ya know?

Duke has made available to audiophiles an integrated multi subwoofer system at a great price. Kudos to him. 
There are three considerations when it comes to accurate bass performance. The woofer itself by which I mean the driver and it's enclosure, the amp driving it and it's integration with the satellites including crossover cut off points, slopes, phase/ time alignment and room control. There are plenty of high powered amps capable of great bass. More electronics are including digital bass management and room control with delay capabilities which makes integration a snap TACT, Anthem and Trinnov come to mind. It turns out the most difficult part to do correctly is the subwoofer itself. Making an enclosure capable of perfectly isolating a high power, long excursion 12" driver is very difficult. Put you hand on your subwoofer while it is playing. Feel that vibration? That is distortion. Any flex or movement of the enclosure is distortion. The ideal enclosure would be infinitely heavy and infinitely stiff. It would be a concrete bunker. This of course is commercially impractical. Designing the internal dimensions of a sealed subwoofer enclosure is easy. Making it acoustically dead is another story altogether. This is where the home hobbyist has a great advantage. There are a slew of great drivers out there and you don't have to worry about shipping and labor costs. You can make an enclosure as heavy as you want as long as it does not fall through the floor. We made killer enclosures with a sandwich of Corian and 1" MDF  Corian/MDF/Corian 2" thick. They weighted 250 lb each and when you put your hand on them you felt nothing. 

davekayc, two drivers operating in phase with each other are acting acoustically as one driver if they are within 1/2 wave length of each other at their highest operational frequency. It does not matter where they are pointed. As an example if the woofers are crossed out at 100 Hz, 100 Hz has a wavelength of 10 feet. If the woofers are closer than 5 feet they are functionally one driver assuming they are operating in phase with each other. I have two 12" drivers pointed right at each other 3 feet apart right up against the middle of the front wall. There are two other identical woofers one in each front corner. They are placed so that the edge of the driver is right up against a wall. They are over twice as efficient this way. They are all acting as one driver, one wave front. Since the woofers are right up against the wall the first reflection off the front wall is exactly the same as the initial propagation so essentially there is no first or early reflection. Since the drivers form an infinite line source there is no reflection from the side walls either. 

I would think that within a certain distance . One opposing driver would either assist or hinder the other at a given frequency. Taking all walls and reflective surfaces out of the equation . Dead of space. Think about a truly tuned exhaust. It reflects a sound wave within to increase power . At some point i would assume the area of an 18” speaker would be affected by the other . In phase. Question is how. 

http://www.lansingheritage.org/html/jbl/reference/technical/1983-subs.htm
This is the design used to build the subs . I built them exactly to design , 12cuft version