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There's no replacement for displacement.

Two 15" woofers to even out your room will be epic.

I'm actually thinking of modernizing my woofer setup. I sold all of my elemental designs woofers locally and will be following this thread pretty closely to see progress

Vastly underestimated amount of polyfill I had left. Where can I get similar stuff, preferably at Walmart or physical location?

I'm guessing this shit works the same? http://www.walmart.com/ip/Olfa-Spinning-Rotary-Mat-12-X12/14003373

Another thing, after reading crazy Magico engineer talk about how screws holding frame to wood is a bad idea, is there any simple way I can get around it? Probably nothing but since I might change drivers/damping I foresee the coupling getting worse

I would go with an ultra long throw woofer. It is all about linear air volume displacement.

For mounting the woofers to the box, I recommend using T-nuts or threaded inserts.

Any models in particular?

Of course mirrors in my house are shaking now. Playing with crossovers in Jriver right now

I wouldn't have gone 15'', very limited choice of drivers at this size. IMO 12'' is the optimum driver size for subs, if you need extra surface, plonk an extra twelver in there. For extra cool points mount two 12''ers in a force canceling config.

Also agree with mr. North here - look for surrounds that look like superbike tires and anvil sized magnets. Class D power is cheap these days, so make sure you've got enough juice to rock. If you're running sealed, consider looking into Linkwitz transform.