First off, I'd recommend playing around with the dither settings instead of leaving them alone. Each one represents a different type of dithering algorithm, each with it's own effect depending upon content. Sometimes you might like one over the other, or one may simply sound more apparent than another on any given source.
As far as what to listen for, don't expect any earthshaking differences. Try different dithers and during playback of each just close your eyes and lean back in your chair and let the mixes hit you as they hit you. Which one just feels better for some reason you can't quite put your finger on? Pick that one.
[SIZE=1][B][COLOR=DarkSlateBlue]Glen J. Stephan,
SouthSIDE Multimedia Productions[/COLOR]
[COLOR=DarkGreen]RECORDING RESOURCES AND INFO SITE:[/COLOR]