How to adapt mpd.conf, to increase the volume?

I have set up a cubox-i2 with Volumio 1.4, to stream Qobuz content from my Android smartphone via airplay (Airplay under Volumio 1.5 didn’t work, only noise was transmitted). The cubox is connected to my A/V receiver via SPDIF. That works so far. But the output volume is far too low. Does anybody know how to adapt the mpd.conf? Changing the mixer options in the Volumio UI didn’t have any effect.