Well... as I understand it you set the crossover level to whatever sounds better to your ears. Generally it is 80Hz some choose 60Hz but most choose 80Hz.
Also, set the sub to 75db if you want it matched to the other speakers... assuming that you are calibrating the other speakers to 75db. Some, including myself, like to have the sub set a couple of db above the other speakers calibrated level. The volume knob on the sub should be set at whatever position gives you close to 75db, once you are close you can adjust it from the receiver to whatever you need.
