All i know is that the lower the Ohm (2ohm or 1 Ohm) will create a more powerful sound, but less clarity hence the subwoofer. Therefore the Higher the Ohm are (4ohm or 8 ohm) will have much more clarity. Usually the 6.5" drive is to create a good mid bass therefore the 2 ohm is usefull for it, as the treble is created from the component tweeter.
I'm not 100% an audiophile, but i think that's what it is from what i learned. Please correct me if i'm wrong.