Ported will be more impressive IRL IMO, the extra output at the lower range of hearing (and feeling) is more appreciated than chasing single digits at the expense of output. I had to learn this the hard way of coarse. My research has basically shown me that the majority of the tactile feel is actually in the 10hz-40hz range anways, and not below 10hz like many try to claim; It's often a futile endevour (or very expensive) to chase single digit bass at a level loud enough to appreciate. For this reason a ported sub at 17hz would be my top choice if you were looking for more excitement. Otherwise you'll need an army of subs and amps, which can work too.
I think you'd like the change to ported from an output perspective, the bass will seem to hit you harder. I've ported and sealed up the same drivers before and the ported is more impressive because of the extra output. My ports were tuned 20hz. I know there was more output from 15hz and down with the sealed, but in real life the ported just crapped in it's mouth. If you want SPL and output and impact ported wins for sure. Moving or placing near field only exaggerates and enhances the ass kicking. If you want to get kicked in the ass, port low and place it close. You won't be disappointed.
If you were starting fresh that would be my advice anyways. But since you already have investiture towards sealed (owning 2+1 operating subs now) then adding two more sealed near field might give you exactly what you want without the need for a bigger cab or complex construction. Sealed is certainly easier to build and can be smaller in size. If this is important, that's a solid reason to stick with sealed. Near field and additional drivers are still going to get you more of what you want, but how much you want would determine if changing over to ported would make sense or not.
If you do it, do it all the way IMO. It's easier. There is economies of scale that will play out to your benefit. It's way harder to build a couple marty cubes, then circle back and build a couple more later. If you do 4 sub boxes at once, it's not much harder than 1. Cut all your wood at the same time, and run all the pieces through your table saw fence on the same setting so all your pieces are identical. Don't adjust the fence until all the same parts and dimensions have been done, that way you get very exact pieces and cuts. If you need to go back and trim 1/8" off and run all the pieces you already cut through a second time to ensure they are all identical that works too. It's slightly more work, but not really. It takes only a few minutes to cut 3 more, once you already set the fence and measure and figure out the cut you need making additional identical cuts isn't too hard.
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."