so I wanted to try going back to a previous version of the iTunes Library.xml file.
I enter Time Machine, and as soon as I back up in time, there are no copies of those files whatsoever (iTunes Library.xml and iTunes Music Library.xml).
Why?
It looks like Time Machine does not backup the .xml files. Have a look at
"iTunes: What are the iTunes library files?"
http://support.apple.com/kb/ht1660
It says:
About the iTunes Library.itl file
This file is a database of the songs in your library and the playlists you've created. Some song-specific data is saved in this file. If you delete the file, iTunes creates a new, empty copy when you open the application, but any playlists, song ratings, comments, or other information you created is lost. The iTunes Library file is only used by iTunes, and is the only one backed up by Time Machine on OS X.
The .xml file contains similar data to the .itl file and is not included in the backup.
Regards
Léonie