If you've followed Al Schmitt for any amount of time you know he's an avid fan of live tracking and placing musicians close to each other when tracking. He like others has said that musicians balance their own levels when they're close to each other and can hear the other band members. This makes less work for the engineer and creates an organic chemistry in the recording.
Al also believes in allowing mic bleed (leakage) when tracking as he says when you use good microphones and the sound of another instrument blends into another microphone it makes the instruments sound larger than life and the leakage dictates how the mix should go.
Note: If you have cheap mics, I'd recommend being wary of too much leakage.
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