It just tunes each wave, heres something I got on xtant.com. Its not the greatest but gives you an idea. And the sub and amps work via a test a few minutes ago at 65 Hz, full volume w/o any tuning.
A dual reflex enclosure can be described as a speaker with a ported cabinet on both sides. This system has the advantage of making use of both the front and the back wave of a speaker, and can attain great efficiency. However, like a vented enclosure, when operating below the tuning frequency of the enclosure, the driver may move beyond its excursion limitations. Combine this with the fact that all information is produced by the ports, so, we can’t hear the distortion until it is too late.