In a room that small, assuming it's not open to other spaces, you're probably better off with a pair of $750ish subs rather than a single $1500 sub. You won't really give up much extension and the importance of the (much) more even coverage provided by dual subs can't be overstated.
If you can stretch a little for shipping, the Hsu VTF-3 Mk5 is on sale right now for $759 each. A pair of those would be my top choice.
If that doesn't work, look at a pair of PSA's upcoming Value 12 for $1099 shipped, a pair of Hsu VTF-2 Mk5 for $1029 plus shipping, or a pair of Rythmik LV12F for $1198 shipped.
If you just want a single sub with good extension, any of the usual suspects would serve you well: PSA V1812, JTR 1200XS, Rythmik FV15HP.
The PSA and JTR you'd have to call/email about, I don't think they are listed on the PSA/JTR website yet. PSA also has a pretty long backlog right now and JTR may or may not have the 1200XS left in stock - it's a limited edition sub made using the cabinet from another product that was cancelled due to chuffing issues, but at $1399 you probably won't find anything close.