I've learned to love Mahler, I start playing his symphonies frequently over the last two years. I just recently played the Reiner/Chicago Symphony Mahler 4th symphony LP from Acoustic Sounds:
Fritz Reiner-Mahler Symphony No 4 Lisa Della Casa-200 Gram Vinyl Record|Acoustic Sounds
Oh boy, that's beautiful! If I see a similar Reiner analog recording for the 8th I will post it here. I also have the Otto Klemperer and Leonard Bernstein Mahler collections, and they are wonderful. The Klemperer set does not have the 8th symphony however.
I think Mahler broke the mold on orchestral/choral music. As someone mentioned above you must listen to Beethoven's 9th symphony, which was controversial and revolutionary in its time. But the most similar music to the 8th would be Mahler's 4th, and several of his other symphonies. He was a serial choral composer, and you should listen to all his works.
Perhaps Mozart's Requiem as well?