I usually keep it very simple with my tapes, either doing nothing at all to them or at most maybe using audacity to add some gain and/or normalizing and that's it.
I'm listening to it for the first time this morning and it sounds ok, but I'd like to 'pump it up' a bit...make it louder, richer (sorry I don't know the technical terms). But this time when I add gain, it distorts. Why is this? An suggestions?
Have you tried Splitting your project in to the individual songs and then normalizing each track(song) separately?
The distortion might be caused by taking the gain above 0db in the mastering process.
If you normalize each song by itself and set the maximum level at 0db that should bring the loudest parts of each song to 0db.
this can help improve volume issues
Listening to the clip you provided it doesn't sound like your levels were to hot so this should help fix the issue.
hope that helps a little.