"Huddled round two microphones". I think this closeness gave a tighter sound to their singing. Look at photos of Young Tradition and The Watersons performing in the 1960s. Even with no microphone, they are standing so close to each other. They were were singing with each other, almost feeling each other's singing. These days, with bands strung out across a stage, each with their own microphone and foldback, there is often not that same sense of singing together or so tightly.
As for the PPM film, what was impressive was the energy of the singing, especially in that first song.
One of the books I've read about the American folk revival (can't remember which and I am at work so can't look it up) describes the way PPM were put together by clever marketing people ... a sort of Monkees for the folk scene!
Derek Schofield