[shorod]

I'm afraid to ask why you're asking. You will not be able to power the subwoofer off of those speakers, even if you used a passive crossover. The power to them depends a bit on which stock audio system you have, probably something like 15-25W for the standard systems. The power to the speakers does not have an impact on using a line level converter as most of them have a "gain" control anyway as well as the amplifier having gain control.

-Rod