X-over that I have had (KMA) or use (IE) is set to around 400 Hz.
I checked the fundamental frequencies over the fretboard, and decided to try to keep these frequencies intact. This led me to around 400 Hz. It seems to work fine, and I can push practically any effect to their limits without losing the clear low end.
BUT the possibility to push effects to their limits means that there can easily be far too much effect. Not only OD/fuzz/dist, but flanger, chorus et al. Not good: it is easy to get too excited of the possibility.