There are two factors to consider with respect to the crossover frequency. One is being able to directionally locate the sub, which you don't want. The lower the crossover the less likely you'll be able to directionally locate it. The other is the capacity of mains to handle low frequencies without audible distortion. The higher the crossover the less distortion there will be. Where it will work best varies with every set up, every room, and every pair of ears, so the only thing you can do is try various frequencies and go with what sounds the best to you.