There is nothing special about using 6 smaller drivers, it's a better value and quality just going 18" subs. Especially if you can place them in different locations instead of all in one spot. I think it would be a pretty cool project to make one that has 6 of the infinity 1262 drivers - it would be inexpensive and would be quite good.
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Using multiples of drivers is always a good idea.
On this forum you are going to find the norm is to use 18's as they are the best value in getting bass for your dollar.
I say model a box with 6 12's and a big pro amp behind it and see what you think. I bet it would be a real conversation starter.
The infinty 1262 driver is cheap and many have used the driver and can vouch for it. They go for sixty bucks each online.
$360 for 6 12's - 400 for a inuke 6000dsp equals about 760. You build the box and finish it all for under a grand.
I would think the Paradigm box uses a good amount of eq to get down to 20hz flat and keep the cabinet relatively small.
Since you are building the cabinet yourself maybe adding some volume to flatten things out would be a good way to having to add a bunch of eq after you are finished. Good Luck!