Robert Plant didn’t always need to have a certain standard in mind when listening to some of his colleagues. Everyone is still trying to copy what he did when working with Led Zeppelin, but the crux of what Plant felt he should do came from him making the best vocals that he could, which suited whatever song he was singing. He didn’t want to belt like he did when he was in his 20s for the rest of his life, and sometimes the best artists for him …