All I can tell you is what George Augspurger told me - and he designed the L100 crossover. He mentioned a method of setting the levels by ear which I thought was interesting:
You can do this with pink noise, or music, as a signal source.
With you seated in your normal listening position (and another person standing next to the speaker), start with both level controls turned fully off. Have the person start bringing up the mid control very slowly. At the point you hear the sound "change", tell the person to stop, and repeat the procedure with the treble control. Again, at the point you hear the sound "change", tell the person to stop. The system should now be set up pretty close to flat.