"Quiet is the new loud" is the title of a 2001 CD by the Kings of Convenience (the Norwegian duo of Erlend Oye and Eirik Glambek Boe). I discovered this CD a few years ago, and it's still one of my favorites. What appeals to me is their good songwriting combined with a certain magical blend in their two-part harmonies. There are some combinations of voices that just work, creating a sound that doesn't really sound like either of the separate voices. This ethereal blend is not the only thing I listen for in music, but when it happens, it's something special, and it certainly catches my ear.
The Beatles sometimes achieved this (though of course they were masters of many sounds), as did CSNY (especially Crosby & Nash - their 3 and 4 part harmonies are smooth too, but it's not the same thing). Simon & Garfunkel probably achieved the most perfect blend (to my ears) on many of their songs - the Bookends album especially comes to mind.
I just discovered an oddly-named duo that has some of the two-part magic as well as excellent songs. I downloaded "Say I am you" by the Weepies (songwriters Steve Tannen and Deb Talan), and although their female/male two-part is perhaps not as sustained and bell-like as S&G or the Kings, it pleases my ears, puts me in that same quietly mellow mood, and just sounds right.