I don't think you need to worry about it to much. They really want someone who can speak to the rarity of the child compared to peers. I chose DD's doctor who is also a family friend. She totally gets dd and is around her enough to know that they way she thinks, the way she speaks, and the things she does is so far beyond her typical age peers. She said she said she wrote that she had never come across another child of dd's abilities in all of her years of practice. If the music teacher seas your ds advanced abilities- not musically, but cognitively, the way he thinks, speaks, the things he understands and says, then I would go with her. If not, if she can only speak about him musically, then I would go with the teacher. Good luck. We are waiting to hear this month. DD's GAI and PRI were both well above the cutoff and her VCI was several points above but her FSIQ was brought down by processing speed and working memory and so was 1 point below.