I am trialing Plex for audio support as GPM will sunset this year. So far, Plex Server, and the Plex app on my phone have been working pretty well.
Plexamp looks great, but it sounds noticeably quieter, worse, and slightly tinny compared to any other audio app on my phone. No combination of settings appears to fix this. It sounds similar to as if a loudness limiter was set on it (and yes, I turned it off in the settings just in case, with no effect). It sounds like if you have a limiter, and it starts clipping - ya know how all the bass is pretty much gone, and it’s kind of tinny/quieter? Except its just that permanently.
I’m using a Pixel 3. I tried on an old Nexus 5x, and it happens there as well, although to a lesser degree.