"This is because jaw movement slightly alters the shape of the ear canal. With IEMs, you need to maintain a good seal when your jaw moves into a wide-open position while singing or speaking."
See, this is the part that I am wondering about. I suspect that most of us are not putting these IEMs through the rigors of a stage performance. So while my jaw may slacken a little, it's never wide open when listening. Do you guys think there may be a "performer" setting and a "monitor" setting?