Where did you get the idea that composers channeled to get inspiration? I've read countless biographies of composers and never once came across a composer speak of channeling in the context that channeling is normally considered.
Perhaps you could give some credible source? Maybe I've missed something in my time!
Or perhaps I should ask in what sense you mean "channeling". I'll agree that music and some ideas come from other planes that artists can tap into but you'll never hear them talk about channeling. They refer to it as "inspiration".
The term 'channeling' has certain connataions to do with receiving esoteric knowledge or knowledge about life from a spirit, sometimes a named spirit. Composers only "hear" melodies and they aren't conveyed by a personable spirit being.