Well Annunaki is the one that figured that port for me... should I still tune it at 20 hz but with a smaller port? That would make the box considerably smaller as well!
Your problem is that port is an organ pipe. It is an open pipe where the wave length is twice the length of the pipe. That pipe has a cross section of 60 sq inches. If you excite the resonance just above 90 Hz it will sound like a lovely wood flu pipe on a historic organ. It will be rich in even harmonics too, with a good deal of output at 180 Hz, 360 Hz and 720 Hz.
Now this is beautiful in a pipe organ but not a speaker.
Now my port has an air speed velocity of 18 m/sec. Would it be better to be 10 or 12 Hz? Sure, but not at the expense of having a port resonance at around 90 Hz.
Even with my model, port resonance is still around 120 Hz, but at least that will allow a 60 Hz crossover point fourth order.
If you want to avid this problem with a resonant enclosure, then we need to model it with ABRs rather than a port, and that will make your enclosure a lot smaller, since you won't have the volume of a large port to contend with. But then I'm guessing you will need two ABRs in each sub, and the expense of them.
Your other option is sealed, which since these drivers are fairly high Qts is in my view the better option for those drivers. Box Pro strongly favors a sealed alignment for those drivers. If it was me I would definitely choose a sealed design with Eq for those. The drivers are good enough to take the Eq, and you have bought enough power to do it. The boxes will be smaller and cheaper to build. A 1,5 cu.ft box gives you a Qt of 0.6 which is really sweet and will not cause the bass to bloom.
I'm certain the designer of that driver had sealed alignment in mind. With two, in a domestic situation you will have far more than enough spl.
