With a ported subwoofer, gaining lower extension is almost impossible, assuming the subwoofer is a good design.

A ported sub is typically "eq'ed" to a flat response (anechoic) to its tuning frequency.

To attempt to gain additional flat response below this will tax the driver, as below the tuning frequency, drivers tend to "flap".

A sealed design is relatively easy to eq - multiple drivers are typically needed, but it's a fairly straight forward process to do so.