you are on the right track with the ported enclosure tuned to around 20hz. for the 25% of the time that you throw some movie stuff at it, it will be a lot more fun than a sealed enclosure by getting you almost 10db more spl around the tuning frequency.
one option is a slot ported enclosure as in the pic, a 2"x15" slot that is a total of ~45" long will tune a 4.0 cubic foot enclosure to ~20hz. i haven't worked out exactly how much volume your cab would have, but that is a back-of-the-envelope possibility.
you don't have to go crazy with bracing. a couple simple 2x2s running from the center of opposite panels is fine.
"Since your primary need is for music acoustic suspension might be the better choice. The cabinet can be smaller, and the sound is often cleaner."
i think what gave ported cabs a bad rap was back in the early days of car audio when really cheap drivers were put in cabs with high tunings (40-50hz or so). as they got down into the tuning region, they didn't have the motor to provide any control and so sounded like arse.
most of the best speaker manufacturers employ ported designs in their top of the line speakers (revel, b&w, meyer, genelec, and so on...) and their primary emphasis is on sound quality. no question that for ultimate ht systems, sealed are better, but that is for different reasons, not sound quality of music.
it is time to start killing off the myths...