I haven't got one of those SPL meters that you can use to set the subwoofer volume correctly, but I think that you can get away without using one. The subwoofer should be at a volume so that it is not noticeable and blends in perfectly with your main speakers. It should only do things when an explosion occurs or something like that. Use a DVD with excellent sound quality (the Star Wars trilogy is meant to be very good) to set the subwoofer volume.
I find that a crossover of 80Hz works well on my system. You can set a higher crossover than this, but this may make it easier to pick out where the low bass is coming from. Most half-decent speakers should maintain a reasonably flat frequency response down to 70Hz, which is why 80Hz is a good crossover to use. If you have very good quality full-range (20Hz-20kHz +/-3dB) loudspeakers then you can just use your subwoofer as the LFE channel.