if the output is a digital S/PDIF signal, it is not a DAC (digital-to-analog converter), but a digital-to-digital converter
even if you can control the volume of that device via volumio, the output will be a manipulated (attenuated) digital signal, basically exactly what you will get with a Hifiberry DIGI with software volume control.
I would suggest you run some tests to determine if you can actual hear that SW volume control is used.
We all try to do everything or, strive, bit perfect, but to be honest I don’t think there is a person in the world who has actually heard it. As every bit perfect stream needs to become analog for us to hear. 
Good idea. I will try to do so. Thing is that I am now happy that I have a (perfect) digital signal from Network/CD to the DAC,Amp,DSP,Crossover in my self build active LS. Hypex FA123 (and 122 modules in my office). No needless multiple conversions. People pressing MQA or other digital studio on LPs. High End Phono Equipement making digital output of this very basic analog information. :).
I use quite decent chassis (Satori). And there is a very, very huge difference between this new setup and my old NAD AMP / 1500 EUR bought speakers before. I had the new self build LS running with analog input from the NAD amp and it was way worse. So it is not only the speakers. It is the chain that makes the huge improvement. Thus I could imgagine that I hear some difference
I use the DSP volume in Roon solves that problem for me, its converted to 32 bit priolr so no dynamic loss form attentualtion. Im runnong a passive preamp now from the output of the Protodac no gain and manright into the XTZ REDGE A2-300 sounds superb compared to my tube pre.
little snippet from my Yputube channel.
Maybe this could be interesting to you.
If there was a virtual mixer available that would do all the gui part of volume control, disregard the final step of actually changing volume.
An extension could be an IR mixer that added the IR control to the end of the gui volume control.