Streamer with variable digital output

WiiM Pro and Pro Plus will both attenuate their SPDIF digital volume and are bit perfect digital transports. The only difference between the two is the DAC section which for the OP’s purposes is irrelevant. I have both Bluesound Node and WiiM Pro that I use digital direct to my DSP active speakers and attentuate volume in the app. WiiM and Node are both Roon ready as well. Another alternative. 

Tune into 8:21

https://www.youtube.com/watch?v=28GQzI5meB0

Even WiiM Mini that does not have coaxial output could pump digital signal into Gan 1 and control volume from finger tips. I'm using the Wii Mini here to address the OP's fear but, in terms of sound quality, I don't recommend it because it lacks bass and sounds lean.

I am still not convinced that any of the streamers mentioned or recommended in this thread, other than the Node, have the VARIABLE DIGITALOUTPUT (not screaming just emphasizing) required by the GAN1. None of the documentation explicitly states that they do.

Ifi Zen Stream works into Gan 1 and controls volume. 

Lumin absolutely does variable volume control on digital outputs and has maybe the most advanced algo available. I have the U2 Mini, it is a streamer only (no DAC) and it has the Leedh volume control. It's excellent.