RE: Digital pre-amp
I don't know who started this discussion but they are right that nothing like what they are envisioning exists. However, very slight variations do exist.
If you look at any AV Pre/Receiver you'll find plenty of digital inputs which can be switched. You can even switch them back to digital and/or pre-out (line level) if you want to bypass the attenuation completely. At that point you've got 2 solutions in one box: A digital switcher / converter and a DAC with multiple inputs.
Now, a "pre" generally means it provides attenuation which means it has "volume control". This can be done on the digital or the analog side. Without attenuation it is just a switcher; like an HDMI switcher. This isn't a popular thing which is why there is limited options.
What, I believe, someone is asking for is a switcher which has multiple USB, Coax, optical, etc. etc. in. But how many? How many USB IN are needed? How many coax? Then, is this simply a switcher / format changer? What is the output and what format is it? Unison USB?
Do you want / need analog out? Does it need attenuation? If yes, now we're back to an AV pre/receiver with multiple digital inputs. I just don't know of anything that provides more than 1 USB input. Again, plenty provide multiple coax and optical inputs.
Anyway, I think such a product lives in a niche market within an already niche market. Certainly not something I'd expect to sell at scale. I, personally, just don't have enough digital sources because my "switching" is occurring within software at that point (swapping Spotify for Tidal as example not MacBook 1 with MacBook 2). I'm not trying to feed a DAC with multiple USB cables. I'm feeding a network connected USB thing from multiple network connected sources; no USB cable swap needed.