"Lark's Tongue in Aspic, Pt. 2" from King Crimson, borrows from Bela Bartok's 2nd quartet, as I recall. Actually, so do The Amboy Dukes on "The Inexaustable Quest for the Cosmic Cabbage". I think LTIA also borrows from Stravinsky's Rite of Spring.
I seem to recall that a number of Jethro Tull songs borrow from the classics.