@rens: the encoder is changing the Volume on Alsa level. To connect the Alsa volume control to Squeezelite just use the Max2Play settings on the Justboom Plugin page: first set up the Amp Hat as soundcard and then follow the description on the same page for the rotary encoder installer. The Squeezelite audioplayer must have the commandline options set to use „hardware volume control“ and „alsasync“. Both are explained on the page.
@rhcp-ng: The Option „-Q“ (synchronize Alsa-volume changes e.g. from rotary encoder with Squeezebox Server) might not be available if you have an older version of Squeezelite. You should try to update Squeezelite on the Audioplayer-Page (advanced settings Squeezelite) to get the latest version. If it still does not work, please post the „command line parameter“ field of Squeezelite Advanced Options and more details on your setup.
The „-Q“ option does only work for soundcards that offer volume control and you need to specify this volume control element! In case of Justboom cards the complete command line parameter for Squeezelite looks like „-o hw:CARD=sndrpijustboomd -V Digital -Q“.