System volume when using speakers

If you are using iTunes, play with the Equaliser settings until you find a level that suits your speakers.

Then adjust your volume using the speakers inbuilt volume controls (if they have one)

 

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.

 

Set system volume to 70-80%, and adjust your speaker volume to a comfortable volume.