I think it was always hinted that the Subaru-sensei was good at teaching and critiquing music. I mean, even if she does have a horrible attitude, even Shiori knew that the concert band was good. Obviously, she's still an awful person but this manga is so good at giving depth to these characters.
Also, that entire speech was everything.
"Then every single song that exists now, and every song in the world that will be birthed in the world, just doesn't want your breath in it."
God.Damn