Feedback from microphones is a common enough issue that can be minimized (but never truly eliminated). It occurs when sound makes an acoustic loop between a speaker and a microphone and usually results in a rumble, tone, or squeal (depending on frequency) that gets very loud. Uncontrolled feedback can actually result in damage, usually to the speakers (and since some of our speakers are hard to reach, we prefer to not let it get that far!).
Feedback occurs pretty easily in the theater with condenser-type microphones or dynamic mics pressed into service as musical instrument mics (where you’ve had to turn the gain up quite a bit). The easiest fix is to turn the gain down and have the mic moved closer to the source you are trying to pick up. For instruments, this is usually pretty easy. For choirs or vocal groups, this can be a bit harder to accomplish.
The first step is to determine which speaker (or set of speakers) is causing the feedback loop. Usually it is the mains (since they actually project onto the front of the stage). If you turn down the main channel fader (or the STEREO master) and the feedback goes away, then this is the loop we need to address. If it is via one of the monitor mixes, then you need to adjust the send to that mix to eliminate the feedback.
The second step is to try to address the frequency that is feeding back. Feedback tends to happen at frequencies that are self-reinforcing. That means that the frequency is decided based on the distance between the speaker and microphone and the length of the wave at a particular frequency. Because audio happens in waves, the frequencies that tend to feedback easily are also multiples of the the primary frequency. For instance, if feedback was most prominent at 250Hz, it will also occur (but usually at a lower level) at 500Hz, 1000Hz, 2000Hz, and so on (doubling each time). Additionally, feedback can occur on not only the direct path between speaker and microphone but also on room reflections, so it can be difficult to predict at which frequencies it will occur at.
Assuming that you know which signal chain is causing feedback and you wish to still have a higher gain on the microphone, then you need to minimize or elminate the frequency at which the feedback is occuring.
For low-end rumble, this usually indicates that feedback is going between the microphone and the room’s subwoofers. For most mics, this is easily elminated by adding a High Pass Filter (HPF) to the input. For most things that you are mic’ing, adding an HPF has no impact on the actual frequencies used by the source since low-end frequencies in acoustic sources is actually pretty rare; you only really see them with large bass instruments (upgright bass, tubas, bass saxes, etc.).