This is how i set up subs with electrostatics.
play some music with heavy bass content and adjust output of sub till you can just hear the sub when switched in and out of circuit. Shut off equip, but dont touch volume control for next step. Then if you have inverted polarity inputs on crossover switch preamp output to that and again play the music with bass content, IF apparent bass level dropped put back, if bass increased your in phase with panels and leave as-is. Listening should be done at listening position. This is fun!