I can't offer technical specifics (even though I am technically an engineer), but this reminds me - I have never posted anything about my experience last month with H-PAS. In "simple terms" it is very much not a simple cabinet design. It combines cabinet design and driver design (which apparently involves some counter-intuitive values) to achieve the desired result (a -2dB point of 29Hz, in the case of the AT-1). There are some specifics on this page.

Back in October, I spent three days at a conference in Boston for work. I was able to work in a short time to catch up with two of the Outlaws while I was in their home turf, including a visit with Peter Tribeman at his office at Atlantic Technology. I had a chance to feed a bunch of discs (some his, some mine) through a set of the AT-1 towers powered by an RR2150. In person, the port is huge - but even with that big cavity it's a pretty handsome tower (if less "exotic" than some stuff that's out there these days). What was really remarkable was how little the 5.25" drivers moved when producing significant bass. I didn't have a tissue for my standard airflow measuring test (great way to figure out return and exhaust grilles and building pressurization), but during loud and low-frequency passages I could feel the air movement through the port with my hand. The port was big enough that I never heard the air movement, but it was clearly a significant factor in allowing the overall system to function. For home theater, most folks would still want a sub so they have that <20Hz thump, but the speakers were able to effectively pressurize the room on their own and achieve great low bass all by themselves.

As a plus, the AT-1 is a well-balanced design - it doesn't just go deep, it also sounds good (clear and accurate) across the full spectrum. The technology (specifically the software algorithm that aids in the design process) sounds like it has some great potential. I'm curious to see what other speakers emerge using it.