How to normalise volume between two sources connected to AV Receiver? (1) Spotify from an HDMI signal (Chromecast) and (2) RPI Zero with Hidizs S9 Pro USB DAC (RCA cable)

Hi everyone,

Audio from my Pi Zero with Hidizs S9 Pro USB DAC (RCA cable) is quieter than other sources, such as HDMI. I would like to normalise it, without affecting audio quality. So in general, I would like the keep the source volumes at maximum, and control volume on from the AV Receiver only.
The Pi is powered by Allo Nirvana (5V 2.85A), I don’t think that’s the issue. I’ve also been told that for the data port on the Pi Zero can output, there is no restriction on taking current from the power supply, so it doesn’t seem like that’s the issue.

I did attach a screen grab jpeg., as printing a PDF didn’t display all the details properly. I think my I’m not very well practiced in using SSH, but if there is a command for Volumio that can spit out a text summary of the settings that are in use, so I can post them here? (Assuming that that’s actually useful to help diagnose the issue?)