I'd probably start at 100 Hz all around and see how that sounds, then move up to 120 or 150 if needed. The best thing to do from there is to tinker with the setting and listen to the same track over and over again until you find what works best to your ears. If you want to get more technical, you could get a frequency response cd (Rives has them) and an SPL meter and actually measure to see which crossover gives you the flattest response.