I use Linkwitz/Riley 24db/octave slopes at 60Hz between woofer and subwoofer and 500Hz between woofer and high frequency horn. The exact crossover frequency and slope is totally dependent on the speaker drivers involved. Linkwitz/Riley crossovers have the advantage of the drivers being in phase through the crossover point, but they must still have their voice coils physically aligned in the vertical plane for there to be the flattest response. This is why some manufacturers use stepped baffles with the tweeter recessed farther from the listener than the woofer.