There are almost none, if you are after an audio-only device.
At this particular moment in time, I personally would only recommend a Raspberry Pi 3 with an additional high-quality audio output module (like e.g. the HiFiBerry DAC+ Pro [or the HiFiBerry Digi+, if you’re after a TOSlink input]).
Then install the Plexamp for Raspberry Pi Release Notes onto it and hide it behind your amp.
For music, this is IMHO currently the best, since it supports loudness leveling and sweet fades (only available with Plex Pass).
However, it requires a bit of tinkering, so its not ideal for newbies.
If you are after a client with video support, you can use any of the currently officially available Plex clients. https://support.plex.tv/articles/categories/player-apps-platforms/
