Dealing with the backwave of a dipole is critical to achieving the best sound from this style of speaker, and since ESL's radiate a LOT of high frequency to the rear (just as much as the front, unlike other ribbon or tweeter based line array dipoles), handling that is critical for best performance.
I run an all MartinLogan setup, design
my own speakers and do fully active crossovers, and have posted a ton of information in related topics over on the MartinLogan owners forum.
Specifically,
the big thread on positioning, treatments etc. for Electrostats; it's 230+ posts chock full of information, detailed measurements and some great discussion.
As for crossover points, I believe 120 is the lowest you should go on a panel, and personally use much higher settings on my large Monoliths.