Re: John Mayer pickups?!?!
He probably wanted them slightly scooped so the guitar doesn't mesh with his vocals, causing false/ghost notes to appear.
When I had a band, we had this one song where the singer wanted to do something kinda weird with the vocal melody where he'd go out of key (in kind of an Alice Cooperish madman sorta way). I didn't like it - thought it came off more like a bad singer than the "effect" he was going for.
I added a string ensemble under it playing the melody (simple and slow 4 note swell type of thing buried under it all), and surprise of all surprises, it straightened out his "vocal effect" so it sounded like he was singing a straight in-key line. He hated it, everyone else loved it, the band broke up :lol:
So, JM may have run across a chord or melody line on the guitar that either stepped on his vocals or pulled them off key, and thus ordered pickups scooped at a given frequency a given amount to counter that, rather than futz with it in the amp or mixer EQ.