What's the recommended level to output from the MASTER OUT terminal without distortion? Can I adjust the output?

If the [MASTER ATT.] is set to [0 dB], output won't be distorted with the volume up to +25 dBu for the MASTER output (XLR).

Adjust the [MASTER ATT.] to set the attenuation level of audio output from the [MASTER] terminal.

For details, see [Changing the settings] in [Settings] in the Instruction Manual.

You can read the Instruction Manual [here].