Extended frequency range is fairly unimportant 99% of the time. Most subs are crossed over at under 100 hz. The exception to the rule would be the use of 8" in the front of the car. In this case, you could cross over the sub higher because localization when in the front of the car is not a bad thing. Crossing over a rear mounted sub higher than 100 hz will make the sub sound like it's coming from the rear.