Shortly after the Dark One Tainted saidin, an Aes Sedai with the Foretelling had a vision that a pool of untainted saidin would be needed. This Aes Sedai gathered a group of both male and female channelers to accomplish this. The only males at the time who were insane were the survivors of Lews Therins' Hundred Companions who went mad on the spot. For the rest of the male Aes Sedai, they detected a difference, but went insane and rotted away gradually. Anyway, this group sacrificed their lives to pull as much saidin through a filter of saidar to purify it, and then they worked together to accomplish the even more difficult task to fix this pool into a single place (there is a quote earlier in the series that it would take every Sister in the Tower to fill one spoon with a physical manifestation of The One Power, just to give an idea of the immensity of the energy necessary).