Barbershop/A capella/Vocal Harmony
 

Could someone recommend some experimental/avantgarde/progressive music utilizing lots of harmonic singing? It doesn't really matter whether truly a capella, or featuring instruments.
I really enjoy the harmonic sound of several voices. I like to listen to some Swingle Singers or Rachmaninoff's "All-Night Vigil", but those rarely keep my interest throughout a whole album.
Zappa in his Doo-Wop-Parody-Moments or Gentle Giant come to mind, but not much else. Time Of Orchids also have some harmonic singing, but they are a little to much on the alternative rock side for my tastes.
There is some wonderful a capella music among the really far-out stuff like Jaap Blonk, Koichi Makigami, Paul Dutton, Phil Minton & David Moss in "Five Men Singing", but there is not much in the way of harmonic singing there.

Any ideas?

Try the SMiLE Sessions by the Beach Boys, idk what your experience is with them but it's a wildly experimental album especially coming form such a huge act in 1967 (yes i understand i'm comparing them to the beatles and hendrix and whatnot, yes they are still even more experimental), it uses harmonies and Brian Wilson genius in a way you never hear quite in the same way again. That said, Smiley Smile, SMiLE's replacement, is also a very strange album in a different way and has some pretty strange harmonies but not as complex as the Sessions.

^
This man has his head screwed on right.

I remember checking the Beach Boys out many years ago, on my neverending quest to check everything out. I sure noticed their greatness, but it wasn't particularly my cup of tea.
But listening to The Smile Sessions now I am really impressed. Truly special and beautiful music. Will have to give their other albums a chance again, too.
Thank you very much!

There's a kid on YouTube, Dan Elias brevig that's definitely worth checking out. I'm a big fan of his dimmu borgir a cappella.

Thanks for the tip, although I like my music in album form and am not familiar enough with Dimmu Borgir to really enjoy this.

I'm so glad to hear you dug it! A Beach Boys reappraisal is always a good thing in my book, I hope the rest does not disappoint. I personally (and this is with the utmost respect for the Boys) can't get all into all of their albums, although they have great stuff on albums I usual wouldn't listen to regularly nonetheless.

Btw, I don't know if you've heard much Animal Collective, they're another band I recommend all the time for instances like these (or any time really, next will be Neil Young), but yeah they use vocal harmonies and the voice as an instrument constantly, it's very forefront and earns them a lot of comparisons to Great Era Boys. So if you haven't checked them yet, I def rec it.

I always thought Animal Collective were just another indie/alternative rock band, genres I'm not particularly fond of (although those might be genres that are to vague and diverse to dismiss them just like that).
As it turns out they do have a quite distinctive sound, nothing standard about it, but I'm afraid they are still somehow to indie/alternative-sounding for my tastes (although I can't quite put my finger on what elements make me think so). But once again, thanks for the tip!

Here's a haunting a Capella performance by Keiji Haino

Animal Collective has done such a wide variety of stuff I'd be willing to bet there's something in their corpus you'd be able to get into.

Acoustic "freak folk"-type stuff, here with Vashti Bunyan singing. Really cute song. :)



And their space-age sound.



But following up on Surrel's point above, I'd encourage many listens of The Smile Sessions.