Hi aadev,
I have one further update for you: the HiFiBerry DAC+ Light is similar to the DAC in the AxiomAir in that it has no built in 'hardware' volume control, but the HiFiBerry DAC+ Light is based on a different DAC chipset, the ESS Sabre. There's one more commercially available DAC board that is based on the same DAC chipset and also has no volume control: the Pimoroni pHAT DAC. There is a Volumio configuration file available for it and it seems like it's been tested already. (Choosing the HiFiBerry DAC+ Light may also work.)
Based on a quick look at the Volumio online documentation, you will likely need to go into Volumio's "Playback Options - Volume Settings" and choose "Mixer Type = Software".
