EQ's should be used as a subtractive device.
That means, they are primarily for lowering certain frequencies in a pre-amp or mixer. (See above photo for a good example of an EQ being used by a knowledgeable engineer.)
The 'smile face' you've all seen on EQ's only means that . . .
the 'engineer' has no clue as to how to use it.
No slider should be very far above '0'! Anything over '0' adds noise!!
This is extremely important in P.A. and recording situations.
Every mix or amp sound has too much of something to start with.
"...Too much bottom end ...", "...too many high's ...", "... mids are honkin' ...".
Use an EQ to lower those trouble spots. Result . . .
Cleaner sound, cleaner mixes, better sound. Simple, isn't it?
EQ should be subtle - not drastic.
The same goes for individual channel EQ on your mixer. Start with all of the EQ knobs straight up (12 o'clock). Make minor changes to tweak the sound of each channel - perhaps as low as 10 o'clock, or as high as 2 o'clock. Between 11 o'clock and 1 o'clock is preferred. Just because the knobs go all the way up to 5 o'clock and all the way down to 7 o'clock doesn't mean that you need to try them there! Again, EQ should be subtle - not drastic. Keep this in mind, and
improve the sound quality of your mixes.