The PreSonus and Focusrite devices are audio interfaces. Just because an audio interface has a volume knob and a digital output, that does not mean the device controls volume of the digital output. The volume knob is usually just for its analog outputs.
I looked at the online manuals for these two units. The volume knob controls volume on the analog outputs of these devices, but not their digital output. As I mentioned in my earlier post, I have not seen an audio interface under $2000 that can control volume on its digital output.
An "audio interface" is not a home audio device. It is a recording studio device that converts microphone and instrument signals into a format your computer can take. On your computer, you use a digital audio workstation (a DAW), like Pro Tools, to record, edit and mix the music you are recording. On an audio interface, the analog outputs are meant to go to studio monitors (so you can monitor what is being fed into your audio interface) or other studio devices to add effects (like a compressor) that eventually go to your computer DAW later in the chain. The digital output is meant to go to your computer DAW, or other studio devices.
The audio interface's job is to bridge devices so they can communicate with each other. The audio interface just sends out a fixed digital signal; your computer deals with playback and volume later in the chain. Although on more expensive audio interfaces, they can allow you to adjust volume on the digital output. (Just like in your home stereo, your record player or typical Blu-ray player just sends out a fixed output; your receiver or preamp deals with volume later in the chain. But some expensive Blu-ray players will allow you to adjust volume on the digital signal they send out.) The digital inputs on an audio interface are usually meant for plugging in digital instruments, like a synth, without doing AD or DA conversion on its way to your computer DAW: it wouldn't make sense to convert your synth's digital output to analog, plug into the analog input for mics, then have the audio interface convert that right back to digital to feed into your computer DAW.
Home audio and pro audio gear live in separate worlds. It is often hard to mix things from both worlds. The Neumann KH120 II is pro audio gear, designed to interface with other studio gear. As an example, the Neumanns only takes XLR inputs, and audio interfaces often only have TRS or XLR connectors -- these are the typical connectors in pro audio gear. Whereas mass consumer home audio gear typically uses RCA connectors. For people wanting to use the Neumanns in a home audio system, the obstacle is that the Neumanns assume a studio environment where you can and do control volume earlier in the chain (like a computer DAW), while the home audio gear assumes a home entertainment environment where you can and do control volume later in the chain (like a receiver or stereo preamp). If you try to bridge the two worlds, there's a gap.
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For the SMSL devices, you would need to check the manuals to see if the volume knob works on the digital output. But these devices are often not designed to do that. With home entertainment source components, the digital outputs are meant for feeding into an external, separate DAC you may prefer to use. In a home audio environment, it would be uncommon for someone to control volume before the DAC: you typically deal with volume after the DAC has converted the digital signal into analog, like in a receiver or stereo preamp later in the chain.
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Some people commented on my original post that you can control volume on the Neuman KH120 II through the network interface. I should have been more clear that I was trying to have a physical volume knob to control volume, when feeding digital signal into the Neumanns.
@staticV3 makes a good suggestion of using a USB volume knob on your computer source.