Another counter perspective,

If you have any woodworking skills and the time to do it I would strongly consider going DIY for the subs. Then you can design them the size and shape you want and finish with the material of your choice for less than it costs to buy premade. My room is almost the same dimensions as yours, 11.5ft x 21ft x 8ft, and two Dayton Ultimax 15" subs in the same ported box under my screen works beautifully for me. Total spent for subs, amp, and building materials was only about $800.
UMC-200, XPA-3, XPA-5, iNuke 3000DSP, M22's for LCR, 4 QS8's, 2 Ultimax 15" in DIY 12cuft ported box