"mic positioning"

The actual sound that a mic gets depends on the exact physical distance from the sound source - moving it a fraction of the wavelength of a single pitch alters that relationship of the phase of that pitch to the phase of all the other pitches, including overtones.

Consequently, just a small distance may have a great effect - but unless you are in an anechoic chamber, you will also get reflections of the sound from everything in the vicinity of you and the instrument. This makes it even more complicated.

You may find it easiest to first try moving the Mics largish distances to get the right 'sort' of blend of all that direct and reflected sound, then try small increments to see if you can detect much change.