It's probably construction. Honestly, you can build almost any box to sound great. With box resonance, that can be fixed by making sure all the joints are glued and adhered properly, you're using 3/4" MDF or better material, and you brace the box internally. I hand built a box with two upfiring shallow Rockfords and the sound is nearly flawless.
Disclaimer: I only did upfiring because of the design of the woofers. The box sits entirely under the back seat of my supercrew and no seat lift was used. Also the woofers are 12's. Otherwise I would have done downfiring, but I need the box underneath the seats cuz my nephew steps on everything, including the seats lol.