Also, a RPi3 runs Linux distros (most notably Raspbian).
What is more interesting for a streaming RPi solution is PiCorePlayer (https://picoreplayer.org/
It has LMS server and a SqueezeLite player preinstalled, so one box solution media server and renderer, headless (controlled from a phone/tablet).
I've been using that for the last week or so to play radio through my old version 1 RPi and a portable speaker over the Pi analog output / speaker line-in. I must say I'm impressed how it just works, even with that ancient Pi. Mind that the sound quality was not what this first Pi experiment was all about. Although, on a portable speaker, it’s nothing to complain about, I used to use Bluetooth by now, and this RPi approach sounds surely not worse. For background music works great.
Also, my experience with playing with LMS vs. DLNA/UPnP during this experimentation is that LMS is a hassle-free solution that simply works, plus it has plugins for Radio Paradise, Qobuz, Tidal, etc. that all work. With DLNA it was a hassle - slower loading of music collection and artwork (the same collection stored on NAS), no plugins support, plain DLNA/UPnP has the playlist on the controller, not the player, which is oh so wrong (can be mitigated with OpenHome via Bubble UPnP, but that also didn't work as elegantly as LMS in my case). To me, the overall DLNA/UPnP experience is very clunky compared to the LMS solution.
In theory we could find apts that output i2s through HDMI or write a module ourselves. It's not going to be easy, but it's not completely out of the question. Could be an interesting project!
See solution #4 here: http://www.dimdim.gr/2014/12/the-rasberry-pi-audio-out-through-i2s/
GPIO has I2S pins, and that is, for what I understand, the solution Allo uses for their HAT boards, where they take that signal and convert it into the USB or S/PDIF.
Now, not sure if Pi-9 does the same, but from how I read its specs and Jason's post on some old topic where this Pi-9 DIY solution was discussed, I'd guess it simply takes the regular signal from one USB port to uDSD DAC board (that's probably why the physical port itself is missing). So, this box leaves a bit of space to tinker with... And, then, again, Omnia is around the corner, and if done right from the software perspective, it will probably beat this solution hands down (I'm worried about the DLNA part, I'd be happy if it could run the LMS+Squeezelite solution on Linux, and still use the OAP chip from Linux OS). I'm hoping my dealer will get a demo unit in Munich to take home, so that we could check it all out.
UPDATE: I’ve just read the article to the end, and it claims I2S on the Pi is not precise enough, and that USB to I2S interface is maybe better to use... Also, comments below the aritcle are valuable bits of info, maybe I2S could be usable after all... Well, a lot to learn there with the Pi.