To the best of my knowledge at present Plexamp is only available to iOS and Android and it doesn’t seem to want to talk to non iOS, webOS or Android Plex clients. I brought up Plexamp on my phone and it can see Plex on my LG TV but not either of the Plex add-ons I have installed on Kodi. If your using apple devices you can send the audio out to Kodi via airplay (assuming you enabled it) and that works fine. On the other hand if that was all you were going to do with it a better option would be to purchase a second hand airport express.
Are we talking about the same thing? Not sure why Plexamp would need to see Plex addons or talk to Plex clients - Plexamp clients, yes. I think it only needs to be able to access music media on your Plex server. Right now I have Plexamp (not Plex) open on my Windows PC and am using it to stream music to a headless install of Plexamp on a Raspberry Pi 3 (I think this needs a Plex Pass). There’s also a Linux version (thought not listed as headless or requiring Plex Pass).
Yes, although it would seem I’m not up to date on the expanded platforms that they have added Plexamp support for (I tend to use it only on my phone). What I was referring to is that Plexamp can connect to a Plex client app and remote control it as a Plexamp client which would allow it to be headless. The same as you are saying your doing with the Raspberry Pi. This capability of being remote controlled by Plexamp, as far as I can tell, is not something that any of the Plex add-ons for Kodi currently supports is what I was trying to convey.
I thought that you had to have a subscription to run Plexamp at all, not just to enable certain features. Am I out of date on that point as well? Looked at their download page and see that they now have some parts of it available without a pass.
I just dusted off my 4K and installed Plexamp headless on it using the same method that is used to put it on Raspberry Pi OS. I used the following guide below with the only thing I had to do different was dealing with the compressed file from Plex. For some reason tar was kicking out an error over the bz2 compression and when I tried to decompress the entire thing in Windows there was an issue with symlinks that are present. What I ended up with as a solution was to just use 7-zip in Windows to decompress plexamp.tar.bz2 to plexamp.tar and then moved it back over to the Vero and tar -xvf plexamp.tar which worked a treat.
Make a mess of headless Plexamp? Those instructions are not going to bork your system. You are not installing Raspberry Pi OS (which wouldn’t work anyway on a Vero), you would be skipping that part and just start with the connecting via ssh section to install Plexamp. This is just adding a headless Plexamp in addition to what you already have. Kodi stays fully functional. The only thing that is going to be different is needing to also install bzip2 as fzinken posted above so the decompression step works without having to mess around like I did. But worst case, yes, you can reinstall OSMC from scratch via the download link at the top of this site. If you decided to install an OS other than OSMC as an experiment and wanted to go back to using OSMC I think that can be an issue if you go that route.