Maybe one of Trent Kynaston's students can chime in here, but I believe he says something about the intonation of overtones needing (or should be) in tune, but I can't remember what it is, sorry.
A sub-tone is basically a...subdued tone. You might want to try a search for this, and I think there is even a featured article about it.
A sub-tone is basically a...subdued tone. You might want to try a search for this, and I think there is even a featured article about it.