14cu ft and $400? I'd do a quad DCS450-18's in that. That would give you the most performance $ for $.
You really can't beat Pro and low-profile cones for ultimate top-end SPL output.
Once you have that done solid, you may find yourself needing far less infrasonic-style bass; which probably goes against everything you've been sold to believe up to this point.
Now none of the following would be as loud up top, but, would have more bottom-end; however you'd still be lacking in >50hz output if you go this direction firstly:
Two FI IB-18's would be cheapest and might squeeze out a few extra db down low.
A future upgrade up from that would be two FTW-21's or two XXX-15's
The 21 would have more output but the 15 would be harder to bottom out, it's more of an easy-button.
It really depends on how picky you are about your bass. Low bass gets very expensive, very quickly.
That's way too much box for an HF-15, you really have to be careful with <30hz stuff with a big box config like that. It's the unspoken gotcha' in the Iron-Law of subwoofers.