Two things guaranteed to mess up a mix at an inopportune time, are a badly set up compressor and a badly set up feedback destroyer. Put the two together, and you have a recipe for disaster.
If you have time, a half decent 31band manual eq across your monitor mixes will make things a lot easier on the ear.
The problem I've found with feedback destroyers left on auto is that they notch out one, then another frequency; then things get a little louder, and the destroyer starts 'hunting' frequencies, never quite controlling them but at the same time scooping the guts out of the signal. Alternatively, they can lock onto a repetitive signal ( synth pads are a particlar pain) and knock them out of the monitors.