This is very basic concept, but one that is often overlooked. It comes from Bobby Owsinski by way of legendary recording engineer, Ken Scott.

Ken always uses the exact same mics whenever he records, and he always EQs at exactly the same frequencies. After watching him work it occurred to me that he wasn't EQing the instrument at all; he was EQing the mics.

Great observation, Bobby. Thanks for the tip!

The entire concept is explained here on Bobby's site.