Efficiency and low end extension. Fantastic SQ. Not so much for SPL even though they get pretty damn loud. They are hyoooj though.
Not every driver is suited for a tline though. My OA15 straight owned a lvl5 in the same "tline." I quotate tline because what you see isn't a true tline. They are quarter waves.
Here's my QW for the OA15 in my full size xcab. It came out to be ~ 9 cubes net.