I Love Music has a thread on sampling of classical works in pop music. Several mentions of The Orb's "Little Fluffy Clouds" sampling of Electric Counterpoint. That work is not a major work in my opinion but it is pleasant and does sample well. Coincidentally, last week, I was listening to The Orb's Adventures beyond the Underworld, and lo and behold, the guitar of Steve Reich, or more precisely, the jazz guitar of Pat Metheny playing Steve Reich, popped up.
In the same thread, NickB suggests...
John Wall, who basically builds ALL his music out of samples of Xenakis, Nono, Schnittke, Feldman etc.
Mark mentions Ekkehard Ehlers...
Ekkehard Ehlers' album Betrieb is based on samples of Ives, Shoenberg, some others.
I tried tracking this down before but haven't actually heard Ehlers. Arguably, Ives was a sampler as well, using the well-spring of American vernacular music.