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First off, given the frequencies being covered, comb-filtering is not a issue since the room reflections and nodes do more to degrade the bass response than having subs spaced out in the room. When the frequencies get below 100Hz, you'd need a huge room for comb filtering to even begin to become a consideration. You might want to research that.
I have a modest, finished basement and 4 Rythmik 12" servo subs in it where I use them as speaker stands for the front L/R speakers. There's enough space to easily add 4 more subs in the rear. 4 subs, though, are enough to cause the concrete floor to vibrate while still playing very cleanly.
As to the weight issue, each of these Rythmik subs weighs in around 55 lbs. All together, they weigh just a hair more than the Perlisten sub. When I need to move them, I don't mind that.
They can easily make it to 20 Hz and, aggregately, do it as cleanly as the Perlisten sub.
As to cost, they, in total, are around $2,200. Adding 4 more would still come in less than half the cost of the reviewed sub and blow that thing away in a room or a lab. Also, the servo system really does it's bit to decrease distortion.
In the end, low bass reproduction comes down to the total radiating area of the drivers. More drivers means more radiating area (two 12" drivers = one 18"). It also means less distortion since the drivers need to work much less hard to reproduce the low frequencies.
Also, there is no woofer on the planet that can take much more than 800 Watts at a sustained level. Most consumer woofers top out around 400. Having 3.5kW driving 2 woofers is a gigantic waste of money. It's BS. It is, though, a great marketing ploy to get the ignorant to spend $9K on a sub (it costs more so it must be better!).
Again, not saying that the Perlisten isn't a great performing sub, but, it's too big, too heavy and too expensive for my tastes.
This is from a bass player and audio engineer.