E-flat major is often associated with bold, heroic music, in part because of Beethoven's usage. His Eroica Symphony, Emperor Concerto and Grand Sonata are all in this key. Also Beethoven's (hypothetical) 10th symphony is in the key of E-flat major. But even before Beethoven, Francesco Galeazzi identified E-flat major as "a heroic key, extremely majestic, grave and serious: in all these features it is superior to that of C."
This is not to say that in the Classical period E-flat major was only for bombastic music with brass. "E flat was the key [Joseph] Haydn chose most often for [string] quartets, ten times in all, and in every other case he wrote the slow movement in the dominant, B flat." Or "when composing church music and operatic music in E‑flat major, [Joseph] Haydn often substituted cors anglais for oboes in this period," and also in the Symphony No. 22 in E-flat major.
For Mozart, E flat major was associated with the Masonics, "E flat evoked stateliness and an almost religious character."
^Francesco Galeazzi, Elementi teorico-practici di musica (1796) as translated to English in Rita Steblin, A History of Key Characteristics in the Eighteenth and Early Nineteenth Centuries. University of Rochester Press (1996): 111