Speakers aimed directly at each other will have crossing wave fields with the resulting peaks and nulls at wavelengths = to the distance between the sources. Solution? Move the 2 sources far enough from each other that it minimizes the effect and avoid aiming them DIRECTLY at each other to further minimize the effect.
I've had excellent results putting the monitors at the front corners of the stage and aiming them towards the center rear of the stage. It sounded great, gave good coverage and reduced feedback by keeping the spkrs in front of the mics.
The frequencies that "hit" your body (roughly 60 to 200 hz) are important in mons, but the deep earthmover frequencies (roughly 30 to 60 hz, should be filtered from monitor mixes so they don't cancel what's coming off the main system. (very low frequencies have long wavelengths so they can more easily cancel each other from surprising distances.
I've never had mono mixs be any problem as long as the input from both left and right channels is present in the mono mix.
That way if the performer pans a signal from left to right at least it doesn't disappear from the mix. Happy sailing!
Chuck
