Monitors are most people’s worst nightmare, and sometimes that fear is justified, but there are some tricks, issues, and considerations to keep in mind that may, or may not, keep you from pulling your hair out during a particularly difficult soundcheck.
These are going to be in no particular order, and have no real relationship to each other.
Any spike in frequency response of a mic or monitor can cause feedback at or near that frequency, since the mic is going to pick up more of that frequency from the monitor than the other frequencies. This is true of off-axis frequency response too, so even a mic that is relatively flat on-axis can have a problematic response curve off-axis. And, to make it even more fun, that off-axis response curve will change dramatically at different off-axis angles. A certain mic may have a sharp spike at 2KHz at 180 degrees off-axis (directly aimed at the back of the mic), but that spike may go away at 120 degrees off-axis (or 100 degrees, or virtually anything). If a particular vocal mic (for instance) wants to ring at a frequency you know is not a hot one in your monitor, the first thing I would try would be changing to a different mic of the same type. That’s right – if you’re using an SM58 (for example) and are having trouble, before you do almost anything else, I would swap it with another SM-58. SM-58’s have pretty lousy consistency from mic to mic.
If it’s still a problem, move the monitor to either side, and aim the monitor toward the back of the singer’s head. Typically, 120 degree off-axis provides the best feedback rejection for a lot of cardioid mics, because there is a slight “lobe” of increased sensitivity at 180 degrees.
Only then would I even touch the EQ. EQs introduce phase shift, which can sometimes cause more trouble than they cure. I would hardly ever boost anything on a channel EQ on a monitor system, especially on lesser-expensive boards. The EQ itself can introduce a slight ‘ringing,’ which only complicates your life further. A good graphics EQ is an absolute MUST. Cheap ones introduce a lot of nasty side-effects at the edges of each band, and you need a very reliable way to boost or cut a particular band without having bad interactions with adjacent bands.
If you are using any compressors on vocal mics in the monitor system, you are asking for trouble. When a singer really hits a note hard, he expects it to be a LOT louder than when he is singing quietly. If it’s not, he will think the monitors need to be turned up. The more you turn it up, the closer you will get to the ‘feedback threshold.’ You may be able to turn his mic up in the monitors, but you have raised the feedback threshold much higher than it would have been without the compressor, with all the problems that can cause.
If you are having problems in the 200-800Hz range, check for obvious things such as a wedge touching the base of a mic stand. If you are having problems with ringing in the 40-200Hz range, it may be low frequency spill-over from the FOH system.
If, during soundcheck, one of the musicians complains that there’s not enough low end in one of the instruments in his monitor, don’t just automatically boost the lows on that instrument. First, call the FOH mixer and have him bring that instrument up in the FOH system. There likely will be more than enough low end spilling onto the stage to please anyone. If there’s still not enough, put more of that instrument in the side-fills. Side-fills are far more capable of delivering low frequencies than wedges, which are usually extremely inefficient below 100Hz. Plus, an abundance of low frequency in the wedges simply eats up power, of which you desperately need all you can get in the wedges.
Also, keep in mind that you have enough power on stage to cause a lot of problems for the FOH guy, so be cautious. If he complains that one of the instruments in the monitors is muddying up his mix, you may have to cut the problem frequency range on that instrument.
There are two clues that the singer’s monitor is too loud (regardless of what he says): He is consistently singing flat, and/or he is not singing as loudly as you know he should. Slowly turn his mix down slightly. He will probably not notice, it should fix both of those problems, and will vastly improve his performance.