In Schnittke's 5th, the 2nd movement was originally intended as a piece for piano quartet meant to act as the 2nd and final piece of Mahler's unfinished quartet. Schnittke enters a dialogue with Mahler by answering subtle fragments from the Mahler quartet with different techniques of Schnittke's musical language, usually in counterpoint with the Mahler, this continue with growing intensity and larger texture until the literal quote from Mahler is played by a piano quartet free of Schnittke's musical context, the 3rd and 4th movements do not use any Mahler quotes, but they are Schnittke's attempts to write symphonically as Mahler did, at least in his late style, not as a pastiche or straight imitation, but more symbolically and elements of form. (Interesting note, his 5th is actually titled "Concerto Grosso No. 4/Symphony No. 5" as it counts as his 4th Concerto Grosso as well, the 1st movement is written in a Baroque concerto grosso form, but with Schnittke's contemporary musical language). Schnittke's first 5 symphonies are extremely rich responses and reflections on musical history, particularly German musical history with an arc going from Bach to Mahler.
Oh, and I forgot to mention, I also actually prefer Mahler's orchestration for string orchestra of Schubert's "Death and a Maiden" quartet to Schubert's original (which is still superb). I easily imagine that Mahler would be hands-down my favourite conductor if we had recordings of him (alas... sigh), at least in his preferred repertoire.
