Lots of stuff I don't agree with in that post DD, actually I feel it's borderline misleading.
1) max output comparisons are useless in this usecase - you won't need it. Even so - no way the sealed Cap S1 is 4x the capability of the sealed DIY UM18-22. Probably a dB or two - maybe three, across the majority of the bandwidth.
2) you don't have numbers to support the nearfield strength of the Cap1 vs UM18-22 since the Cap S1 hasn't been measured by data-bass. If you take a look at nearly all the 18" sealed - most are within a couple db across the spectrum of the UM18-22. The built in DSP EQ on the Cap S1 will revert towards native sealed FR as the subwoofer goes to max tilt so the max spl figures comparison will be more equitable to the native FR as you approach that level of output.
2)You can't compare tactile feel of caps farfield vs. sealed nearfield with the dB numbers. Even though that's not what I was discussing. I've owned a pair of cap 4000 passive placed far field and I LOVED them (still one of my absolute favorite sub setups) -- yet the pair of them farfield can't shake my body like a single 18" nearfield sealed.
Tactilely --- Not even close between nearfield and farfield. Yes - if you put a cap 1400 behind the seat it'll knock your teeth out. But my favorite part of the nearfield is the tactile deep wobble feel -- that, you don't get with a horn or ported -- and heck - that I don't even get from my farfield sealed. (if it wasn't for the sealed nearfield experience on my basement slab theater room - I wouldn't give two toots about sealed extension)) I've said it before and I'll say it again --- for tactile feel I'd take one of my sealed 18" directly behind my seat over the other seven sealed up front. I've experimented early on with this in my room - that was my subjective takeaway when I had it setup 7 and 1.
My nearfield sub experiences
Ghorn at Dougs, Othorn at Doug’s (two of the best horns available) and a similar experience of an Orbit Shifter Pro at cvinfig’s house but off to the side. (one of the best horns available)
Cap 4000 passive at my house (one of the best ported available)
Sealed 18” SI 18 at Sheldon’s, and Austin (popalock) as well as sealed 18” UM18-22 at my place (all inexpensive sealed 18” drivers by comparison) --- and a Dayton 15” HO sealed at Nelson’s (KCNitro07).
In my particular room I'd be perfectly happy with a pair of ported Captivators or Orbit Shifters up front and a sealed 18" subwoofer (any decent sealed subwoofer - SI18, UM18-22 - whatever) behind. After experiencing quite a few rooms with 'inexpensive' sealed subwoofers in large quantity - or nearfield --- even the inexpensive sealed subs get the job done. Nearfield sealed remains my personal favorite - not the loudest, not the most tactile (horns and ported are more tactile nearfield), but I like the wobble/pulse feel the sealed create nearfield with the low frequencies that otherwise I can't detect in my room on a basement concrete slab with the sealed or other sub alignments farfield.
Subjectively speaking - - Tell @carp
that my UM18-22 nearfield don't have enough output. They were at my house fairly recently for a movie night - we watched Mad Maxx. Carp asked me to turn the bass from the nearfield down at one point cause his teeth were chattering. jedimastergrant agreed. In this scenario I'm talking about feel - not max dB.
If it's not enough tactile feel move it closer --- the proximity makes a BIG difference, even a few inches makes a big difference in my experimentation. I may not be as big of a bass hound as some of the guys like d_c or n8dogg -- but I've never been accused of having lightweight bass preferences around these parts --- that I'm aware of anyway. Knowing Derek's preferences now after visiting his house, he doesn't usually run his bass hot from the mains at all. I'd wager any of the sealed sub offerings would meet the bill. Again not discounting the Cap S1 - just saying the other is probably sufficient.
Not all of us are blessed to have a suspended wooden floor theater room that bounces the couch around like an amusement park ride. Your subs are farfield and you get that at your place. I can't compete with that tactile sensation on my concrete slab without the nearfield sub --- unless I'm prepared to have my ears ring for a week straight after every movie session.