I just tested the MPD player myself to make sure there are no new errors in our newest version of Max2Play. However, I could, unfortunately, not replicate your issue.
What parameters are displayed in the „Soundcard Settings“? There should be a selection in the „Sound Device“ line indicating the hifiberry amp.
What power supply are you using? The Amp’s need a separate power supply since they power passive speakers, the Amp itself and the Pi (no normal Pi power supply should be connected).