The best subwoofer shape - cubic?

My last build was a closed subwoofer using a Dayton Audio Reference RSS265HF.

I like it a lot, but I want to test a setup where I combine the RSS265 with two 10" passive radiators to compare.

It seems like all subwoofers with this setup have a cube shape. Is this the optimal design, or is it used to make the subwoofer as small as possible?

I have found that the passive radiators should be installed on opposite sides of each other to cancel some resonances.

476497639_1662564938003718_1873370181416621862_n.jpg

My first closed build



476498210_1162379075475890_881103839368249739_n.jpg

An idea I did play with for a ported setup - could skip the port and add a passive radiator on each side (adjust height..)