For speaker crossover frequencies, ideally they shouldn't be fixed in the human speech range: 400-4000 Hz. But for practical purposes, this is not often feasible.
It's of most importance to fix crossover frequencies which are suitable for the chosen drivers. These drivers have to be a good match. Passive crossovers have to be well designed for a smooth frequency response, using quality capacitors, not the electrolytic type often used in commercially sold, even expensive ones.
Spoken and sung words should be intelligible. Unfortunatley, it can be technically challenging to retain intelligibility, when recording or amplifying the voice. In this article, we present some facts on speech intelligibility and, most importantly, how to retain it.
www.dpamicrophones.com