Final answer:
Gustav Mahler composed the 'Symphony of a Thousand' and was a renowned conductor of the New York Philharmonic.
Step-by-step explanation:
The composer of the 'Symphony of a Thousand' is Gustav Mahler. Mahler was indeed an extremely successful conductor and composer who became the music director of the New York Philharmonic. This symphony, his Eighth, is known for the massive number of performers needed for its full rendition, hence the nickname.
Other composers who were known for pushing musical boundaries include Berlioz, Strauss, and Wagner. Like Mahler, their music often challenged the audiences of their time with new concepts and dramatic expressions.