> For micing an acoustic guitar in a space with large active speakers what
The total gain (in dB) needs to sum to <0 dB, eg. if you have two
amplifiers, one attenuates at -6dB (* 0.5), then the other must be < 6dB.
The thing you have to work out is what the attenuation/gain of the
monitors-air-mic chain is, once you know this (just play with it until you
get feedback and work it out form the other settings), you will know what
other levels are safe.