Volumio allows to select the best mixer preference according to your audio chain.
First:
Go to SETTINGS menu
Go to PLAYBACK OPTIONS menu
Here, you have 3 mixer choices:
Software: mixer algorithm is demanded to Volumio OS
Hardware: mixer algorithm is demanded to the streamer board (suggested if you are using Volumio Primo without external DAC)
None: mixer algorithm is demanded to an external DAC/AMP
Note: on Volumio devices where the volume is managed by the device itself (such as Primo), the Mixer Type option isn't shown. To get a fixed output, set the volume to 100 and control it from your amplifier.