This are my assumptions, and i am not an audio engineer.
This midrange is not designed for bass even if has lower fs. 30hz
The low fs come from the very lose suspension, believe it or not a loose suspension means eficiency, in a way, because it lowers the qes! Also making it good for ported, making it even more efficient.
Thats why they have such low qes qts.
This means that the coil doesnt have to be denser to achieve lower qes, instead the coil can have thinner wires to achieve a bigger bl (more total coil in the gap) better motor force controll. And with the loose suspension mechanical looses are very low.
This gives it very low distortions