amixer sset 'PCM' 96%
amixer sset 'Playback Boost' 0%
This is done to prevent clipping/distortions that start above 96%. Sadly many applications directly use ALSA's PCM control to manipulate the volume. One example would be mpd/volumio.
When sitting on the couch or on the table to aren't allowed to increase the volume above 96% and have to get up to increase the volume at the amp. You'll probably end up having to explain this to friends that visited you and want to control the music.
Can't you simply remap the max volume within the driver so that whatever control you end up using you can't get your hifiberry to start clipping? If this can't be done in the driver, where would be the right place to set correct bounds for the soundcard: Kernel? ALSA? Any detailed infos where i could read up to gain some insight?