After using a sound system, always turn the master volume down to zero. The master control governs the volume of all channels. If the volume is up when you turn a sound system on, there is a risk of loud, uncontrollable bursts of sound, especially if certain components aren't properly connected. Mitigate this risk by making it your policy to turn the master dial down as part of your turning-off process. Always check the dial is at zero before turning on, in case the previous user hasn't turned it down.
Equalization, or "EQ", is the process of boosting and reducing certain frequencies to create a sonically pleasing result. There are two types of equalizer: graphic and parametric. A graphic equalizer typically has 30 slider dials that control the level of specific frequency bands. A parametric equalizer is more rudimentary, typically featuring approximately five dials that control the level of more general frequency ranges, such as bass, and treble. Before you equalize the sound, set each dial to the center. This gives you a neutral starting position.
In each sound system application it's imperative to test the system before use. Testing involves setting the volume levels, the equalization and adding effects such as compression, where necessary. To accurately analyze the influence of these factors, listen to the sound system from different places. Start by standing centrally between the speakers, halfway between the speakers and the mixing desk. The central position allows you to hear the stereo image of the sound. Move backward and listen for any frequencies that are too loud, or are "peaking" as you move. The shape of the room, material of the wall and even features in the room such as windows can influence the behavior of sound waves. Only by analyzing the sound system from a range of places can you accurately asses the sound quality.
Head-room is an audio term. If you maximize the volume of each sound, there is no head room to increase. Never push a slider dial up full. Rather, leave room to increase the volume of an instrument or intensity of an effect. A sound system in an empty room sounds different when the room is full of people. To this end, you may need to adjust the sound system once the room is full. For example, after a band sound-check, the singer may appear sufficiently loud. But when the audience arrives, the vocals may appear too quiet. If you haven't left head room, you've got no way of increasing the vocal volume.