In the beginning, back in the late sixties and early seventies, the monitor systems were put into place to combat the problem of the slew rate. It was a minor, very slight, delay between the note you were playing, and the time it took to get to the PA. But, it was enough to throw the best people way off.
I will agree that most people seem to hit notes flat rather than sharp, which given the choice is preferable. For years the Dead has a woman sing with them, Donna Jean, who on most of the live recordings was always flat, but they stuck her between the drums where she could not hear herself think, much less sing. The last few times DSO played the Fillmore she has shown up, and with the in-ears, its amazing - she is spot on every note, which is how she sang in the studio too.
And yeah, for an acoustic guitar, you don't hear it right, the sound hole is facing away from you. Electric stuff is different.
P.S. What kind of mike are you using, the standard for vocals is the Shure SM58, which most singers like because its good, without being too, too good.
