2L Coke bottle gravity bong.
And no... I never did that. Sure I got high. Even made my own Honey Bear Bong at one point.
But that doesn't really make a difference about how one can expect 8" Subwoofers to perform and in what circumstances.
You should learn about Speaker Design. Even the most basic read through, you will learn that no matter how many driver's you stack up, you will never lower their possible extension.
All of this is a balancing act. 8" Drivers are rarely effective as what I would consider a true Subwoofer. My mains, using 8" Revelator Woofers perform down to 25Hz. They aren't Subs. Just a well designed Woofer.
Even Salon 2s with their 3 8" Drivers per cabinet and F3 of 23Hz cannot hit as hard at 20Hz as a good Subwoofer could.
Wiring 2 12" drivers in parallel means nothing if they are low sensitivity Drivers, for example. Sure you will get a boost to SPL, regardless of the Driver Sensitivity...
But so much more comes into play.
This is the thing about all the different gear on the market. Every Speaker is a (hopefully) carefully crafted balance of compromises. Some get it right, Others don't. One can deliver high output with a small size... another may be big and dig low, but not play as loud. *shrugs Which is right? Both.
A Lifestyle Subwoofer like the 3000 Micro and the little KEF are probably great for a small apartment or little listening room where deep bass isn't the goal. Compare that performance to the SB 3000 for just $200 more. With the 13" Driver, you get an F3 of 18Hz and I would expect... greater output and more headroom.
All of this is rudimentary and comes back to the most basic thing: What is the User's intention? If you are looking to fill in below some small monitors and not disturb your neighbors, you may well want the Micro. If you want to watch a movie at reference level... that Micro will leave you wanting.
Ultimately, the only right answer is User dependent.
I've been very careful to not say this thing is cr@p. I may not personally take it seriously, but there are people that this will make happy for various reasons. I would likely never recommend it, but for the people that it will work for... it may well satisfy.
Yet for every person that a Micro may satisfy, it is almost as possible to point to a person buying multiple NSW6021s for a Dev or Marty Build and expecting every inch of pant-leg flapping they will get from that investment.
YMMV, of course.