I currently added a subwoofer to my system. My speakers, measured with a SPL meter could handle 40Hz before rolling off. Initially I set my crossover at 60Hz. But when I did a SPL measurements at various Hz I got a dip in the 63Hz to 125Hz. The flattest response I got was when I set the Outlaw crossover to 100Hz. So this is where I keep it. What does all this mean? I don't know. But if you have a SPL meter and a disc with various frequencies, you will be able to get a much more accurate idea where the best crossover point should be. Real world measurements are better than technical guesses.
my .02cents