Joel writes regarding Mahler's possible early influences:

>This makes me wonder if there is a body of such pieces by lesser
>composers of the latter 19th century by which Mahler was influenced,
>or whether he specifically knew those of Schumann.

I have "heard" Mahler in the expanded orchestrations of Beethoven's late
String Quartets, such as Bernstein's with the VPO on DG.  (Also in some
of Schubert's late Quartets).  You might want to try these out!

John Smyth