@T-Rex Honestly, this sub really doesn’t model real well in a ported enclosure. It is pretty surprising, as they used to model similar to the ultimax 18, however this version seems to have gone away from that. I would personally do sealed with this sub. You are gaining very little as far as low end performance goes and most of that should be made up by the room. I wouldn’t even use a linkwitz transform unless necessary (which I don’t beleive it will). Ince you add some stuffing your anechoic F3 will be between 28-30Hz. However your room will add gain down low in frequency or could even do some eq if necessary.
Ported, you really woudl want something closer to a 12 cubic foot box ported to 20hz. THey way it is tuned currently you are loosing a lot of the low end efficiency that is typically the reason to go ported. The Mx15 seems to perform better ported (however you loose the size of the 18).
So for me, I would still either do the MX15 ported or the HT-18 V3 sealed or a very large ported box.