The line splitter isn't useful here, it splits line level signals not speaker level.
Are all the speakers in the same room or separate rooms, and do they need separate level controls? At best a 2 channel amp is capable of powering 2 zones, if they need more zones then need more amp channels.
For those speakers an amp that generates 200-250w/ch at 4ohms and is 2 ohms stable would suffice, so an RMX850 would be plenty.
It goes before the power amplifiers so if they had a multi channel amp it could have been used as a volume controller for each speaker. You could still use it with a 2 ch amp of course but there is no way to get the same amount of control.