From Wikipedia

The Genius Of Beethoven

Episode Two

Between 1798 and 1802 Beethoven finally tackled what he considered the pinnacles of composition: the string quartet and the symphony. With the composition of his first six string quartets (Op. 18) between 1798 and 1800 (written on commission for, and dedicated to, Prince Lobkowitz), and their publication in 1801, along with premieres of the First and Second Symphonies in 1800 and 1802, Beethoven was justifiably considered one of the most important of a generation of young composers following after Haydn and Mozart. He continued to write in other forms, turning out widely known piano sonatas like the "Pathétique" sonata (Op. 13), which Cooper describes as "surpass[ing] any of his previous compositions, in strength of character, depth of emotion, level of originality, and ingenuity of motivic and tonal manipulation". He also completed his Septet (Op. 20) in 1799, which was one of his most popular works during his lifetime.

For the premiere of his First Symphony, Beethoven hired the Burgtheater on 2 April 1800, and staged an extensive program of music, including works by Haydn and Mozart, as well as the Septet, the First Symphony, and one of his piano concertos (the latter three works all then unpublished). The concert, which the Allgemeine musikalische Zeitung described as "the most interesting concert in a long time", was not without difficulties; among other criticisms was that "the players did not bother to pay any attention to the soloist".

While Mozart and Haydn were undeniable influences (for example, Beethoven's quintet for piano and winds is said to bear a strong resemblance to Mozart's work for the same configuration, albeit with his own distinctive touches), other composers like Muzio Clementi were also stylistic influences. Beethoven's melodies, musical development, use of modulation and texture, and characterization of emotion all set him apart from his influences, and heightened the impact some of his early works made when they were first published. By the end of 1800 Beethoven and his music were already much in demand from patrons and publishers.