The M300 features line inputs which require a higher signal than a microphone (in most cases) can provide, when connected directly.
Also the M300 outputs are line level, a signal too high for most mic inputs on mixers.

Ideally, you should either:

1.
Connect your M300 to the effect loop of your mixer using the send/returns on the mixer - or -
2.
Connect your microphone to a mic preamp capable of providing a line level signal to the M300 inputs, and then connect the outputs of the M300 to the line inputs of your mixer.