I'm new here but i'm an owner and I look for the disbelievers. I want to know when a 15db drop from 30Hz to 16Hz is falling off a cliff. The last time I looked a ported box should drop off @-24db/oct not -15db/oct. The second graph is with room gain not at the port as the top graph. -12db roll off will be flattened by room if it is not too big. Typically sealed boxes achieve a -12 db roll off if they are designed well.
I doubt that anyone drew these graphs so it is what it is on group delay and why the subs blend so well. This is no resonant box and the low mass aluminum (50hz free air) driver gets the job done quickly in the sub bass region. The guy who introduced this is using it for HT discarding 4x12" drivers in a suitable box. Give him a break if you haven't experienced it.