I'm pretty sure that PlexAmp, backed by Plex Media Server, is the pinnacle of music discovery, within whatever you've got in your library. I've got that serving an enormous library that I've compiled through my whole life, but there's no reason (well, other than legalities) you couldn't feed this through the *arrs.
The discovery algorithm works based on fundamental metadata, additional data pulled from Last.fm (e.g., "related artists" and "popular tracks"), as well as its own acoustic matching algorithm.
This arrangement obviates the SD card, and also any other external syncing (PlexAmp plays live from the server, or has its own scheme for downloading to a local cache).
I guess that gets to what each of us means by "discovery". I think you're talking about the existence of a given artist/track. But for me, with a sufficiently large library, it can be difficult to remember what you have, and what would go well in a given context. So PlexAmp's "discovery" helps me to navigate through that large library to find and re-experience stuff I'd forgotten about or overlooked.
The discovery algorithm works based on fundamental metadata, additional data pulled from Last.fm (e.g., "related artists" and "popular tracks"), as well as its own acoustic matching algorithm.
This arrangement obviates the SD card, and also any other external syncing (PlexAmp plays live from the server, or has its own scheme for downloading to a local cache).