Plex server streaming to Plexamp here. Currently handling around 50k tracks all stored on my NAS no problem. Soundiiz supports Plex, so converting Spotify playlists over to Plex is pretty straight forward, provided you have the songs.

• Plex pulls down it’s own metadata, so if you’re a tagging freak like me, you’ll have to check the “Prefer local metadata” on your Plex server.
• Smart playlists are a little cumbersome. They’re actually saved filtered searches. Not intuitive at all.
• No HiRes - if that’s your thing. (on iOS, not sure about Android)
• Plexamp has a separate EQ for each bluetooth device on iOS, but it can’t differentiate between wired headphones using an adapter.
• It does save music or playlists to the device for offline playback, but they’re captured within Plexamp. You can’t play those offline tracks in any other app (might be possible in Andorid, but the filenames will be random and idk about metadata). I have not run into an offline download limit, like in the old days.