This is a characteristic of second-order (12dB/octave) crossovers - at crossover, the sealed box highpass function has a phase shift of +90 degrees, and the lowpass has a -90 degrees phase shift. That 180 degree phase difference makes them cancel each other, leaving a deep hole in the room response at the crossover frequency. By reversing one of them, they are in phase and the hole is filled in.
Of course, this assumes the sub has a second-order lowpass crossover, and it follows the same red/black phase standard as the fullrange driver. Since the described subwoofer has not been in production for many years, one would probably need to experiment with phasing if a current production sub is used.
