Thanks, guys!
That was pretty much what I was thinking was going on, when I said that maybe MusicBee was creating a "bad XML." I didn't mean necessarily that MB was creating incorrect XML code (though, that was a possibility), but thought it was probably more likely that there was something in the perfectly correct XML that it created that iTunes just doesn't like.
My next challenge is that the XML file is over a million lines long, and I'm not sure what all to search for that iTunes might consider to be an "invalid" character. Not sure what to search for there, other than possibly some foreign language characters. I'll see what I can figure out.