Make sure your sources are good and level differences between guitar/keyboard patches are evened out. Once your source is good, make sure your operators are setting up the channel pre-amp gains to light the -6dB light (I like this better than 0 – more headroom and no sound quality penalty), and then not touching them once set.
IEMs are less forgiving than wedges are, and can reveal a lot of things – pitch issues, vocal technique issues (distance and position to the mic, needing to pull back more or less during loud notes, etc), sound operator technique issues, etc. They are worth the adjustment, though!