I've always been frustrated that my wife's iBook way outshined my MacBook when I played iTunes -- even with iTunes and internal volumes set to max. Then Leopard comes, and I did a Quick Look on a music file in the Finder (select the file and hit the Space Bar). I noticed that the Finder played the song much louder than did iTunes.
Eventually, I figured out that if in iTunes you enable the equalizer (Window » Equalizer then click the On button), and set the Preamp to its max, the volume now matches the volume of Quick Look in the Finder, less any adjusting on the equalizer. I already had the equalizer on, but the Preamp was set to 0.

