For it to make any sense, Plexamp would have to support “exclusive mode” and bitperfect audio like Audirvana or Tidal. Otherwise, everything would just get re-sampled to macOS native 48kHz before being passed to the BTR5 via USB.
Ironically, this is how “hi-res” Amazon Music works - since the app in Chromium based, it can’t do exclusive mode or send bitperfect audio to a USB DAC. So even though you may be paying for lossless/hi-res, 44.1kHz or 96kHz tracks just gett re-sampled to 48kHz in the software before passing to the DAC.
In order to actually get lossless audio to a USB DAC, Plex would have to create custom drivers for each operating system that bypass the native OS audio drivers.
You can do this in MacOS, Windows (via ASIO), and Android (via USB audio API), but it takes a lot of resources to do it.
You are better off sticking with Audirvana if you want hi-res output and then firing up Plexamp when you want to listen and don’t care that it’s downmixed.
I’ve asked about this before and unfortunately audiophiles like us are too few and far between for them to dedicate to resources to let us listen to hi-res audio in Plex
