Even cheap subs can make SPL, so that's not a biggie.
Good Low End Estension is a must for HT, but that's not what sets apart a top performer.
Looks are not related to performance, and those can be polished later, so that's not it.
I'd say its the right combination of tactile response and low distortion when hitting high SPL at low extension, with tactile response being the one factor if I could only pick one.