its something you are doing. i clamped my amps and im getting 1750w per sub. i did a ton of testing, at least an hour of burps and bass races. my subs didnt even get warm. room temp at best. sssooooo, my best guess would be that you are clipping. remember this could be at the amp, hu or even the song itself. however it could be the box. i just looked at your build really quick and i think your port looks a little small for 14 cubes. maybe too much pressure is building up inside the box and they arnt moving as much as they should.
how are you so sure that you arnt clipping?