You have Volumio on rPi4 right? And you connect to the D-6 DAC via USB, right? I suspect it may be a firmware limitation of the D-6's XMOS USB interface, or simply that the XMOS USB interface has a USB vendor/product ID which Linux (and Volumio) does not recognise as being compatible with hardware volume control. The best way to find out is to go to the Volumio forum - quite frankly, that should have been your first point of contact, not audiosciencereview.In the Volumio DAC settings
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with the SMSL d-6 the HW mixer is not visible.
In the meantime, there's a simple solution - configure Volumio for NO volume control, and use the D-6's remote control for volume adjustment.
I'm fairly sure welder911 is using Volumio under Linux, so Windows drivers are not relevant to his situation. Rest assured that Volumio setups, if properly configured, may be considered "super serious about SQ".if you're super serious about SQ you should install the drivers which enables ASIO ...