See my notes under "my Altec A10" - which uses 515b's
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The theater configurations get their low end from the huge cabinet surface area (AKA "wings") and the distance from the audience. 3 dB down at ~90 Hz isn't that bad. From what I remember from my acoustics classes is that the coupling (with air) roll-off (de-coupling) begins at the wavelength that matches the circumference of the radiator.
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A 15" radiator has a ~4' circumference, which puts the 3 dB down decoupling point in the 200 Hz .. ~250 Hz range. 4 such radiators will drop the de-coupling point an octave - which matches the Thiele-Small prediction quite closely.
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There are 5 ways to increase the radiator size - horns, ports, passive radiators, more, and wings (a form of horn for large diffraction situations - which require at least 1/2 wavelength to fully develop).
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The guiding rules of all engineering are: anything that works is a compromise and Mother Nature is a bitch.