I have come to the conclusion that SW/Main crossover should be set based on what your particular SW can do rather than what your Mains can do. Any decent Main speaker ought to get down to 40 Hz or so, but doing so may adversely affect the Mid bass range, 70Hz up to about 200 Hz. Give the Mains a break, and set the X/O to 80-100 Hz. Try it that way first. If you don't think the SW can hack it, then go lower.