I think the Beatles sound fabulous together, but I'm rotten at deciphering the harmonies. I don't know, for instance, why George sings one verse on "All My Loving" instead of John. They just sound like Beatles to me. And
that sound is virtually unreproducible.
That said, all three of the main Beatles vocalists have very unique voices. I keep remembering this quote by Jerry Lee Lewis, when the Beatles came over so big in America in 1964. "Their voices all blended together, I knew they were going to be huge, and I hated them for that!" John's voice is unique in rock'n'roll. Ditto Paul and George. Together, they forged this wonderful, perfect harmony.
Er, I have no idea if I answered your question. But it's lovely to think about! Cheers. *has another glass of wine*