Apparently, the teachers knew that Atsumu’s tone is a bit problematic but then, Suzuka already knew the answer and wanted them to realize it themselves rather than telling them. That is actually better since it is Natsu himself who volunteered and decided to do it.
From what Chika mentioned, it is like thinking out of the box. Rather than thinking of how to resolve the issue wherein they cannot change a certain aspect because of circumstances, they think of changing other possible aspect/s. What was thought to be a defect can actually become an asset when reinforced with another thing.
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Hehe, Natsu wanted his contribution to be low-keyed yet appreciated but unfortunately for him, everyone already knew about it. I think this will make everyone know this side of him more and like him more compared before. Everyone is touched by what he did and was amazed over this possible resolution to the problem.
From the last part, apparently, that and what Natsu told Atsumu had tremendously improved their playing of the piece. So, ah, what is left is resolving the theme/what they want to convey with the piece, right? Anyway, I guess they will be pretty much ready just in time for the Nationals.