I'm pretty sure the cap is ported, the UM kit is likely sealed.
Nothing is stopping the OP from making ported boxes, that is, other than a $400 roybi table-saw and router/bit set.
If he uses a kit, then all that is needed is: wood glue, 4 wood clamps, a bag of wood screws and a power driver/bits. (Which he probably already has.)
Usually when it comes to bass, the more cone area and coils, the better. As it helps reduce excursion and heat, which reduces distortion.
Having two amplifiers connected to two different breakers helps draw more power, which helps get louder when needed. The fans help cool things down, although the downside is a bit of fan noise.
(2 breakers are 3db louder than 1 breaker, of equal size.)
The NU's are fairly quiet, similar to a modern day projector in eco-mode, if even that loud...
If you want to hear just how loud some of the stock amplifiers can get, take a look at this video on my YT channel:
The NU's are comparable to the EP, or slightly less loud than that.
A Crown iTech under load literally sounds like an F-18 going vertical.
It's fans are a million times louder than an NU.
That CA amp is like an SR-71 with afterburners on, it has 4 4inch fans constantly going at high RPM.
Comparable to an FP on start-up. Good grief!
For example, you can't hear the two nu6kDSP's powering 4 UXL-18's in this vid as there is music playing at a medium SPL:
If you don't want fans in your amp, you'll have to fork out the money for the speakerpower plate-amps. As that is pretty much the only good fanless amp useful for sub-duty, stopping short of perhaps a pair of A/B McIntosh 2kW monoblocks or A/B Emo XPR-1 space heaters.
The cool thing about 4 sealed 18's is that they will reach down to ~5hz, where as a ported sub will drop like a rock starting as -3db @ tuning (because of its HPF and port/cone phase issues.)
Not sure about you, but I happen to enjoy the 7hz tone in BassILoveU and Irene\WOTW scenes... of course it won't be "uber loud" at those low frequencies, but at least it is reproducing it moderately-well.
It is fairly trivial to disable a HPF filter and crank the port velocity so high it levitates your carpet, but that doesn't mean it is loud...
i.e. ^^^this, and videos like this
That's like 2000% cone distortion, the PA cones are pushing out to where there is -20 BL
Unlike that,
videos like pop's, notnyt's and mine, we are moving things with
ACTUAL sound waves.
You know... actual
SPL /
DBs. Not "20hz below tuning" box-fan particle velocity turbulence effects.
The bass wants out of the room and it is desperately seeking a place to escape...
^^^ The OP will have about this much displacement / SPL. This is 2 21's and a LMS-18 with about 8kW. He'll have 4 18's and 12kW burst; so a comparable amount...
The distortion on my HzHorn is like 0.25% @ 134db @ 16hz. All the distortion you hear is from the mic clipping to death.
That's just 1 LMS-18 @ only 400watts, corner-loaded and horned. The iTech -10db below clipping on just 1 channel.
4 UM-18's horned could almost kill a person. hehe
The only downside is that the UM-18's needs a HUGE horn box, that's if you go that direction.
About 40% bigger than mine. Otherwise it will sound horrible. Probably best to just keep it sealed or ported (unless you have a huge barn-sized room or are a real bassheaded basshead.)
Horns are not a good starter sub. They are hard to design and build; and really only suitable for movies-only IMO (or ugly music like dubstep/rap etc.) Depending on what your tastes are.