Sonos volume levels?

Recently aquired my sonos system.Two play 5's set up as stereo pair and a smaller play 3 upstairs. with all my cd's now on the imac and using napster.Now I am very impressed with the overall performance but at a recent get together at my place required a certain ramping up of the volume which although was addequate I did feel it could of done with another level or two.Are the volume levels set or is there a way round this?