I can recommend these two linux audio players:

[gmusicbrowser][1] and [Quod Libet][2]

- support huge libraries (mine is 60.000 tracks with more than 100 GB)
- GTK+ based
- free and open source
- support (auto-)playlists
- support most common audio formats
- GUI is customizable (lots of preconfigured layouts)


  [1]: http://gmusicbrowser.org/
  [2]: https://code.google.com/p/quodlibet/