i mean again it may be your box thats stopping you, which it could be 1000000 different things lol. i still say for those zv3's like smaller boxes that one seems alittle bit big. i mean thats pretty much 6 cubes probly after displacement each sub and again the sundown site says they do good in cubes net but i may be wrong.
i know it should be louder then what you did. i did a 147.6db with 1 hdc3 18 in 5.36 cubes with a shotty refurbed planet audio bb2400.1 @ 1 ohm and it only clamped like 500 rms(
you can add the bracing, and tweaking the position of the box, also you could resin the inside of the box and see what you can get out it, if that doesnt work id start with a different box, you def have good equipment tho i dont see any need in changing that. i gained like a full db or close just by glassing the inside