To adjust the volume of the 851N, Digital Pre-amp Mode needs to be enabled within your units settings.
This feature will then apply to the analogue outputs of the unit only. This is because the amplification occurs after the DAC within the units circuitry.
To enable Digital Pre-amp Mode on the 851N please navigate to Settings > Audio > Digital Pre-amp Mode.
Once enabled, you will be able to control the volume of the unit and in turn, your entire system via the control know on the front of the unit, the remote control or the StreamMagic app.
Please note, the muting function of your mobile device may also mute the 851N.