After updating to iTunes 10.x, many users have reported that their music playback sounds bad (canny or tinny). If this occurs, quit iTunes, open Audio MIDI Setup (/Applications/Utilities), and verify the format for your output device is 44100.0Hz. Apparently this setting sometimes mysteriously changes after updating to iTunes 10.x.
Apple also defaults the setting for Sound Enhancer to on with iTunes 10.x, which some users do not like and say it actually makes music sound worse. To turn off Sound Enhancer, go to the Playback tab in iTune’s preferences.