Have you tried Apollo? I just posted about it a few days ago, and it has great sound quality and richness. It's good enough that its my current default player. Check out the responses to it here. It also has my thoughts on iTunes player, I'm sure Napster won't be as good, but I have yet to try it.
The biggest negative for Apollo is the fact that it loads the playlist into memory. Not great for large playlists, but I did link to a plug-in that helps with that.