Well why is the EQ there in a PA speaker?
Easy it is simply there to let you adjust the speaker to a not perfect room. Aka in a standard environment you would leave all the EQs in the middle position (including the FREQ) and do the soundcheck from your mixer.
Now in a not so perfect world these are the things that happen mostly:
– Bass gets sucked up by some carpet or curtains etc. You need to adjust this with the BASS EQ.
– High Freq gets reflected too much and it sounds like a tin can (tiles in the room, many windows without curtains,…) you need to adjust with HIGH EQ until it sounds right
– Mids are different, the worst thing that can happen is that your mids trigger some resonance in the room. Now the thing is that those resonances are room dependent and not always the same frequency. So what I do is cut the mids totally when I hear a resonance (usually some kind of purrrrring sound or clattering sound) then go through the frquencies with the MID FREQ knob until the resonance is gone. You just hit the right frequency. Then I turn the MID EQ up again as much as I can without having the resonance sound.
Now some sound techs will tell you that even those things should be done from a good mixer and they never touch PA speaker EQs, but our world is not perfect and as DJs sometimes we use no PA mixer that has adjustable mids…
Hope that helps you.