I didn't read the build just looked though it. the ports looked like 4" circles so that is what i went off with the four ports now you only have 50 sq in port area. I think it said it was 2 18's so going off that you should have a min. of 150 sq in port area so that could be your problem.
And for the different tricks and thing I have always seen it done with lower notes. I got 4 Fi BTL 15's on 8000 watts and can barely do a little one but the box is tuned higher for SPL.
Best thing to do is build multiple box and until you find one you like