The manga has lost its way, it seems to me. It had been at its best when it was basically about an ordinary school teacher learning to have fun with music and dealing with petty school politics and low level personal drama. But it took its own premise too seriosuly, and for a while now it's been epic, edgy and humourless like a bad shounen title.
The brass band teacher is a fantastic villain, but how does she measure to a shadow cabal leader literally murdering people left and right? He's in an entirely different realm, and it happens to be a fairly boring realm. Every time the stakes get raised, it just cheapens the ordinary life context that made this manga fun in the first place.