This can occur if a channel is not patched to an input source that has a preamp, such as an effect return, or if the channel is not patched to any input source at all.

Here's how to patch an input source (e.g. a console input or a digital snake) to an input channel:

1. Press the PATCHBAY button beneath the M-300's screen.
2. Press F1 [INPUT] to select input channels.
3. Press F3 [REAC A], F4 [REAC B] or F5 [CONSOLE] to select REAC A network, REAC B network or the console inputs as the audio source.
   
4. Use the cursor up/down buttons to select the input channel.
   
5. Use the cursor left/right buttons to select the input source.
   
6. Press ENTER to patch the input channel to the input source. A checkmark will appear in the patchbay box and the preamp knob, 48v and PAD will appear for the input channel.