The crossover frequency of the factory sub can not be altered for the most part.
It is a bandpass designed box which means only a small band of sound can exit the box. The rest is not audible. So you could only lessen the range of sound that comes out of it. You could not make it play any lower. You could only make it play less high.
You can not power that sub from a head unit as its external wires are for signal input only. There are no exposed speaker wires. You would have to crack open the enclosure and then disconnect the leads to the factory speaker from its internal amplifer ( its a 6" speaker, not actualy a sub) and replace it with wires from the head unit. This would be monumently worthless though as it will eat up 1 of the only 4 channels of the head unit and will not power the speaker aswell as the internal amp did. The internal amp is capable of the same power if not more then the head unit is in total to all of its channels.
Also, the alpine head units can not lowpass the speaker wire outputs. I belive only the bass engine pro head units could do anything like that and that was really a bandpass not a low pass. Although I am not sure if it only applied to the head units rear RCA outputs and not the speaker wires.
If the sub is not what he is looking for, then its time to get a new amp and subwoofer combo
