Denon AVR Volume Control is a plugin for Volumio 3 that allows you to control the volume from Volumio.
This plugin is completely work-in-progress and will likely cause problems with your Volumio installation. There is still a lot to be done before it even reaches the testing phase.
This plugin is in no way affiliated with Denon/Marantz. Any implementation I've managed to wangle is from trawling these documents available on the web: https://rn.dmglobal.com/usmodel/HEOS_CLI_ProtocolSpecification-Version-1.17.pdf https://www.denon.com/-/media/files/documentmaster/denonna/fy21avr_denon_protocol_v02_04062020.xlsx