The two drivers are out of phase from each other at 32 Hz. That is why there is so little gain in output below 50 Hz. 100Hz 125Hz cont. The third group is the horn loaded type speaker system. The DS-2P falls into this category.
Horns have the advantage of more then twice the efficiency of reflex systems.
However, horns need a large mouth area to reproduce low frequencies. A traditional circular exponential horn design to reproduce 50Hz is a large and unwieldy thing. A mouth diameter of 8 ft. is needed to couple 50Hz to the air and the exponential taper rate has to be chosen well below 50Hz. The DS-2P was designed to be arrayed together to create the large mouth necessary for concert sound and to be modular for easy truck packing. The DS-2P was designed to operate with a minimum of two cabinets placed together when on the ground or four cabinets together when flown in order to reproduce 50 Hz.