Here are some that I would like to share to start this thread:
1. "Shame on the blind men who took Beethoven for a deaf man!" Wilhelm von Lenz (1809 - 83)
2. "They want me to compose in a different way; I could, but I must not." Anton Bruckner (1824-96)
3. "This boy will cause us all to be forgotten." German composer Johann Adolf Hasse (1699-1783), about the 15-year old Mozart.
My comments:
Concerning the third quotation: What a prophetic thing to say!
Concerning the second: Genius recognizes itself!
Concerning the first: Talk about metaphoric blindness and deafness!