For compressing before the A/D conversion, use just a touch of compression.
1.5:1 or 2:1 ratio, fast attack (about 5ms) and medium or automatic release.
Usually, 3 to 6 db's of gain reduction with these settings you won't make the vocals seem so squashed down.
You could also turn on the limiter to catch those sudden peaks that occur during the music recording process. Remember that theres no automatic compression setting so sometimes tweaking through trial and error may work best in order to get the type of musical sound that you are looking for.
