Is there an optimal volume for OS X when using external speakers? Or does it really matter? I'm not thinking about doing anything special, just listening to music or playing games.
Naturally it's always 'whatever sounds best to you'
If your speakers are ran through a complete line-out then you shouldn't have any way to adjust osx's volume, only iTune's volume. If you aren't, then you're probably double amping and the speakers aren't being as efficient as they could be.