Should be well under $2500 shipped, plus taxes unless he's outside conus. For that matter the VFT-3 MK5 has almost identical perfomance to the VTF-15 MK2, with just slightly less deep bass performance (17 hz vs 16 hz), but either will play well into the infrasonic range below what we can hear.
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$1769 plus tax and shipping for a pair of them and $909 for a single. Of course I would recommend a pair for such a big room.
If you do go with a single the Captivator is a pretty good candidate. It's a lot of sub but I do agree with Eppie about the advantages of going with dual 15s. As he said, you'll have more cone surface area, equivalent or better spl and the advantages of having duals to help even out the response.
I like what you did with the D9s! I totally believe you that it helped round out your bass and made everything sound fuller. With the way you hooked 'em up I can see it working out as long as they weren't bottoming out or anything. Nice way to get a little more life out of them, lol. Wait til you get some proper subwoofage that can hit true infrasonics in there tho. You get everything set up right and it'll take to your system to the next level.