It sounds crazy, but one of the loudest setups in town a few years ago was a standard cab silverado with 2 15"W0's and a 600w legacy amp. Guy has super small sealed boxes and those things got crazy loud for that many "watts". They were single 8ohm ran into 4ohm mono. I eventually bought them for cheap from the guy but they were never the same outside those little sealed boxes.
I guess what I am saying is that some super shallow 15's in a small sealed box is what I would go with. Maybe not those subs and sure as hell not that amp, but you get the idea.