My knowledge base is limited to 20th-century avant-garde classical, but lately, I've been enjoying a few noteworthy albums:
Harry Partch - The World of Harry Partch (1969) featuring the sidelong composition,
"Daphne of the Dunes"
This was an enjoyable follow-up to my first Partch record,
Delusion Of The Fury - A Ritual Of Dream And Delusion which was released just two years after TWoHP.
I confess I still haven't cracked the cover of his masterwork,
Genesis of a Music from 1946. I really need to make the time to read it.
John Cage • Christian Wolff (1963) was another rewarding purchase. Side A is Cage's "Cartridge Music" - sounds created from the manipulation of turntable cartridges. This was his first electronic work and is really a significant milestone for collectors of electroacoustic recordings. Side B features three pieces by composer Christian Wolff -
"Duo For Violinist and Pianist," "Summer for String Quartet," and Duet II for Horn and Piano." This side is far more pastoral and gentle and serves as a lovely complement to Cage's composition.
And the third recent purchase was
Panorama of Musique Concrète: Works by Henry, Schaeffer and Arthuys (1956). I suspect you are already familiar with this recording, as it appears on numerous musique concrète essentials lists.
I'll name one more LP you might enjoy - this one from the Folkways Science sublabel, issued in 1957.
Various – Sounds Of New Music showcases compositions by John Cage, Henry Cowell, Vladimir Ussachevsky, Henry Jacobs, and others for a wonderful collection of avant-garde music. But it was track 5 on side A which made me pick this one up - Edgard Varèse's famous
Ionization.
My apologies if you dismiss these as "old-hat" avant-garde. Modernism and Futurism have come and gone, and you may be seeking novel concepts in new music. But I consider these important works and find them to be wonderfully enjoyable even sixty years after their composition.