I think I understand what you are trying to do. I don’t know your exact setup but, let me offer this.
The way it works is, the WiiM device is a network streaming device with chromecast and airplay. So it works functionally the same as a chromecast. Typical use case. Top level explanation.
- Connect WiiM to your network
- Download WiiM app to setup and initialize the WiiM Device
- Connect the WiiM to your DAC via optical or Coax.
- DAC to Amp, then Amp to Speakers.
Now that the WiiM and your hardware is setup how does it work with Plexamp? Magic. Kidding aside, once the WiiM is setup you simply need to go into the Plexamp on your phone, => select player => tap on the player named WiiM, this will then cast to the WiiM, the WiiM will then send to the DAC etc…
To be clear the WiiM does not download any apps onto it. You don’t need the app store. The WiiM is using chromecast or airplay to send and receive data and play music. This might not be what you had in mind, but that is how this kind of setup works. I personally use both the WiiM for other streaming things, but I have a dedicated Raspberry Pi 3B+ as my go to device for Plexamp. The Raspberry pi is my Plexpoint. (Plexamp Endpoint)
For example, in your case, you can test out the concept using a cellphone/tablet. If you have Plexamp on your computer, and Plexamp on your phone/tablet. You can use Plexamp on your computer to select music, hit play, then select the player you want, your tablet/phone in this case, the music will stop playing on the computer and start playing on the tablet. And you can continue to use the computer to search for songs or playlists to play next.
Once that is established and working, take that one step further if you have a USB Dongle DAC plugged into the tablet/phone, and the dongle DAC is plugged into an amplifier and speakers you will get the music.
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So with this simple case, you could use an old cellphone running Plexamp, and a Dragonfly, or Helm Bolt, or Chord Mojo, and you have a simple setup there that is easy to maintain and use. But a more permanent solution would be something like a WiiM or Raspberry pi, or chromecast, device etc… The Plex server computer where your music is stored, is similar to the roon core you mentioned, and the raspberry pi’s with headless plexamp are the roon ready endpoints, like the bluesound node or as I like to call them Plexpoints!