sounds like eq adjustment on the overflows might be in order
Not necessarily.
the signal for the mains comes from the LR from our Allen & Heath GL2400-32 board. the signal for the overflow areas comes from the mono mix (LR combined).
with a nice balanced mix achieved in the main auditorium, in the overflow areas it sounds like a different mix. especially the vocals are noticeably more prominent.
Currently, the mix in both the main auditorium and the overflow are essentially the same mix. In the auditorium there is a lot of direct sound from the instruments and not so much from the vocals. (Without amplification, instruments tend to drown out vocals.) To balance out the mix vocals are amplified to a greater degree than instruments so they will seem to be at comparable levels. In the actual mix being fed to the FOH amplifiers and speakers (as heard in the headphones) the vocals will be noticeably stronger than the instruments.
For those seated in the overflow area the FOH speakers become a part of the perceived "direct sound." If you turn off the overflow speakers you'll find that the mix is good, but not loud enough -- hence the need for overflow speakers. Turn the speakers back on and you're getting that perceived direct sound plus the FOH mix (which already has hot vocals) and the vocals will seem too hot.
The answer is that a different mix is needed. If a post-fader aux send channel is available it can be used to feed the overflow area with a mix ("in the headphones") where the instruments and vocals are more equally balanced. Since it's a post-fader send it will track changes made to each channel for the FOH mix and can for the most part be "set it and forget it."
But there still may be EQ adjustment necessary on the overflow feed to compensate for room resonances in the overflow area.