I have read as many posts on how to set the sub crossover and I need a little help with my particular settings. I don't know how to find out what the -3db point is for my speakers. My front speaker frequency response is 55Hz - 28Hz. My rear speaker frequency response is 50Hz - 20 Hz. I understand from other posts that the sub crossover should be the next highest setting above the -3db point. My crossover is in my receiver and there is no crossover on the sub itself. What is a good starting point?