The problem with much 20th Century so-called classical is that it is like modern jazz but without the rhythm.
I know what you mean. The first time I heard the World Saxophone Quartet or Art Ensemble of Chicago, I was a bit surprised. A lot less melodic and more atonal than Ellington or Basie. But all they did was take the extensions of Bird & Trane to their logical conclusions. Same with Bartok. You can hear the beginnings of what he did in some of the work by Brahms or Beethoven. Or even influenced by Black & Tan Fantasy. They just kept working it further down the line.