I don't think it's the chain so much as the arrangement. His voice is mixed loud relative to the instruments, much moreso than is common today. Plus, through most of his singing, there's only bass guitar, kick drum, and high hat in the background. So nothing competes for the piece of spectrum his voice is in.
The instrumental fills, things that could clutter the song (acoustic guitar, piano, and fretless) all come in the spaces in Simon's vocal part .. So really, it's just good arrangement and production. His voice itself sounds mostly untreated to me ... Uncompressed, open, with just some reverb to put him in the right space.