That impedance curve looks a bit like that of a speaker with an aperiodic port that needs a bit more stuffing. Those rsl cabs are interesting. They remind me somewhat of Atlantic Technology's HPAS cabs. Although AT's goal was extension, both have unconventional enclosures that feature some sort of novel hybrid/truncated/reverse taper transmission lines with side chamber, and from the looks of it they must damp the active driver a bit differently than by-the-numbers bass reflex. I would actually have been surprised to see the classic symmetric impedance peaks Gene expected.
I have been looking at this. It is not a reverse taper TL. If it were the start of the pipe would be widest. It is more like a horn that is too short. There is an expansion box, throat, and another expansion, just like a horn would have. Then there is that port.
The impedance curve looks like a that of an incorrectly tuned QB4 box. The 3 db point is around 100 Hz and the roll off is fourth order like a ported box would.
I'm not at all convinced that internal contributes anything but bracing. I would like to see the comparison of the same drivers and crossover in a properly deigned QB4 box. My feeling is it would be superior.
The speaker sounds good because the crossover seems spot on and the drivers are perfectly integrated.
As for the bass loading, it seems a mish mash of a too short TL with expansion and taper of a horn and then an attempt as mass loading with a port. Pending further information and data from the company, I'm not impressed with this loading arrangement.
The Speedwoofer on the other hand at first glance looks more promising. Although F3 is fairly high, it is rolling off second order, like a TL would.
I would like to see more measurements of that, and details.
I have tried various configurations over the years to try and reduce the real estate of TLs, including mass loading them with ABRs. The only ones any good were labyrinths. I suspect if that GL4 where a labyrinth it would be much more promising as far as the low end is concerned.
The price would be higher as there would be a lot more internals. However I think the 3db point would be lower and roll off second order, with more low bass, and low Q to boot.