Asking this mostly out of curiosity and a desire to learn and understand what is happening. I have my theories-but curious what others think.
A few weeks ago we had a special conference-special speakers, lot's of visiting churches, all round pressure to be at the top of our "game". First speaker wants to use the lavalier-it is set and works great on our pastor, but be projects well and generally has good technique and a clear, easy to mic voice-the opposite of this speaker. On top of that, Pastor is on the platform-wanting to hear the message clearly in the monitors. A bad scenario to deal with.
I am not mixing, but I take over and back the monitors off some-but it is the mains that are ringing. I cut a little 250-300 where it is ringing and boost a little in the 1500-2000 and now I have a clear, easy to listen to sound-but pastor wants the monitors up. I am able bring them up until he is fine with them without creating any issues-then I bring the mains up a little more and feel I have a really good sound-but pastor tells me we lost the monitors. I haven't backed the monitors off-and I never "fixed" them later-I know they were working and just as loud as when he said "good enough"-but apparently raising the mains made them "go away" where he sat. What gives?
Mains are QSC K12's, monitors are QSC K10's mixer is a QU-32-not that it should matter much. Room is very reverberant-designed and built before PA's were a thing.