If it was me I would get the measurement gear since you have a sub you can test with. One thing I don't think has been mentioned is that with the seating close to the wall you are going to have some very strong response there and probably strong modes due to being near a boundary. Testing with your existing sub might help understand it, and if you had another test sub to put in the corner that would be ideal. One sub on the front wall centered between the future subs will be similar response to the two subs, enough to test. Then you will be able to see the effect of the side sub.
I have two hs24 in sealed boxes on a 14k in a room 18x14x8. There are six 2x4 two inch thick yellow fiberglass board absorbers, mainly for first reflections. I don't listen crazy loud and run the bass a little hot, and some bass eq. It's plenty of bass, if it's wanting anything it's more subs to even out the response more, or more near field. If you just want max output and you listen loud, ported makes sense. If you mostly listen at -10 and run the bass hot, the sealed boxes will be plenty. It plays EOT intro at -10 without sweating which would probably be reference level if the bass wasn't hot. It's plenty loud. Loud though, isn't the real issue, these subs are super capable. Even response is the real challenge. Bigger rooms are better, they take more power and subs but will have smoother response in general, and open plan seems to be great, for response if not ultimate volume.
Once you get measurement gear and start working it out you'll understand better the challenge of getting even response across the seating. And once that happens, you might appreciate smaller boxes that allow more flexibility of placement. In our family room I have two uxl up front and two 15" on the sides near the surrounds. It's tempting to think that the 15 can't keep up, but in reality they provide great smoothing effect, mostly because they help excite one room mode more evenly across the seating, which makes it easier to eq out. The chair my wife likes is near a wall and had a killer mode. So I either turn down the bass, or found that the side subs then cause that mode to be excited all across the seating, which means when the audyssey runs it nukes that everywhere. Wasn't an approach I would think of but it was super effective.
Here is each sub alone, both subs, and post audyssey. Lightest blue is front, next light blue is rear, both centered in room on front/rear wall, dark blue both, red post audyssey. I have a wicked issue around 85hz which the waterfall shows but since we're moving it isn't gonna get addressed more than likely. You can see also the modes that swing the response like crazy. Ultimately I think the thing is that you can have such a challenge to get good response without the added issue of ported/sealed phase issues.