The default setting is +0 db. If you "toy with it" you can just put it back to where it was.
As for what it does...on the surface it seems to change the volume. It also makes a horizontal line in the EQ go up and down. And it distorts the sound if you move it up the slider. So...based on all this my guess would be that it affects how much power the EQ changes affect the sound. i.e., at +10 DB, sliding the 16K slider up will multiply 10db by some value associated with the slider's position and affect that frequency's loudness accordingly. Hence, the reason the sound becomes distorted is because your speakers (or in my case, cheap headphones soon to be replaced by better ones

) are being given frequencies at power levels they can't handle. Or since this is software, the sound is being altered past what the soundcard can deal with. Something like that.