All these answers are good, but to get an exact answer we need to know the exact model number of the sub. I agree line level is the best way to go but different subs do different things to the line level pass through to the main amp. Some may use a crossover set at 80, 90 or 100Hz. Some may not even have a line level pass through. For integration it is best to consider what is happening to the signal all the way through.
Cheers,
MaxCast
Cheers,
MaxCast