OK, this is really interesting. The version released two month ago had a bug that prevents iTunes from recognizing SoundCheck values for some m4a files. But the latest version seems to work fine, which means the bug was fixed at some point during the last two months. So I'm in the middle of updating SoundCheck values with the latest version, and so far the values are correctly recognized by iTunes without its own volume analysis. I'll report back after a full test.