The mniDSP can DSP down to 10hz if I recall. The iNuke stops at 20hz and so you have to fart around to make it extend lower.
Not sure if the miniDSP has a limiter, but that is something you'd have to level match yourself to the external device. Output voltage from the amplifier calculated as per ohm's law.
The inuke 6k would allow for 2 UM-18's to be powered in the future, where as a 3k might struggle.
The old school Class-H amplifiers are very inefficient, often 40-60% at-best, and are about 8 times heavier, the EP4k for example.
Class-D only produces the amount of power needed as it is needed, therefore it is higher efficiency and cooler running, and can make use of smaller heatsinks because they don't generate 50% waste-heat.
The inuke's are Class-D and the Crown 6k and iTech is Class-D, the cheaper Crown's are Class-AB/D hybrids (last I checked, which was 10 years ago... LOL)
PowerSoft K/X and SpeakerPower are also Class-D.
It makes no sense to have 10kW of waste-heat to make 10kW of bass, that would require a 240V 100A breaker just for 1 amp LOL. That's why you don't see old school amps rated for much more than 2-4kW.
MOST music and movies are very bursty/dynamic in nature, making 10-20kW from a bank of caps short-term isn't too big a deal, hence why the K20 is possible.
Pro amps are designed mostly for live music where power is only required in bursts (guitars, drums, vocals.)
Generally the RMS power is roughly half the peak power, many of the higher powered designs have auto limiters to avoid popping breakers or exceeding the thermal/ampere limits of the guts as the fans hit max RPM.
Many rock concerts also play the numbers game, if you throw enough cones and amps at the problem, the SPL at Distance X will be solved. So you'll see 30-100 18's and 30-100 amplifiers, powered with multiple diesel 18wheelers.
Ported is louder than sealed roughly from tuning until about ~40hz.
If this is for movies, 15-17hz is a good tuning point, and 25hz if it is just music.
Ported boxes requiring a HPF at the tuning frequency.
Most sealed boxes start rolling off around 30hz, but the rolloff is less severe than ported, and stays in-phase for the most part, unlike ported.
This always you to boost the ULF flat to 3-5hz. With 4 to 8 18's those frequencies will start to become usable.
Sealed eats up more power and thus is likely to cause melting and bottoming sooner than ported would. No free lunch.
When it comes to making bass, you basically want the biggest cones you can afford, that have the highest power-handling and excursion, and as many of them as you can afford.
db/w/m is important above 40hz, excursion is important below 40hz.
It takes about ~32 watts to push 32 18's to 1mm, but it takes ~3200watts to push 1 18 to 32mm, and both are the same SPL.
and then what happens when the array goes to 3mm's? (or 20mm's?) EXACTLY!
The single melting and farting in seconds/minutes, and the array is bass-for-days with zero distortion the whole time!
Thus: if/when you want it louder, just add more. 6db per doubling.
Doubling the distance is -6db in free-space, in a room it is slightly improved (when not placed in a room-null.)
Groundplane is half-space, a wall is 1/4th space, and a corner is 1/8th space. (Corners are loudest.)
The difference being room-gain, which starts when the wave bounces off 1-or-more of the 6 surfaces and/or when the wave no longer fits in the room.
Sometimes moving closer to a subwoofer makes it sound weaker because of a lack of these additional reflections and/or being in a null zone; or conversely moving away makes it louder for the same.
Nearfield is when your ear is closer to the cone than the closest surface(the floor), then your SPL will be at maximum (minus room-gain though).
Therefore the loudest/smoothest place for a subwoofer is often at head-level directly behind the golden-seat firing into the back of your skull point-blank range and delayed several ms to match the LCR's. Too bad it is not very practical and low WAF!