Question: What German composer wrote a letter in 1770 addressing Lucrezia Aguiari's ability to perform in C sporano acuto?

Context articles: Leopold Mozart
Johann Christian Fischer
Lucrezia Aguiari
Recommendation letter
Beethoven and his contemporaries
Blond Eckbert
Cassiber
Friedrich Witt
Two Little Pieces (Yoshimatsu)
Heinrich Schütz

Johann Georg Leopold Mozart (November 14, 1719 – May 28, 1787) was a German composer, conductor, teacher, and violinist.
 Mozart is best known today as the father and teacher of Wolfgang Amadeus Mozart, and for his violin textbook "Versuch einer gründlichen Violinschule".
Johann Christian Fischer (c. 1733 – 29 April 1800) was a German composer and oboist, one of the best-known oboe soloists in Europe during the 1770s.
Lucrezia Aguiari (sometimes spelled Agujari) (Ferrara 1743/1746 – Parma 18 May 1783) was an Italian coloratura soprano.
 She possessed an unusually agile voice with a large vocal range that spanned slightly more than three and a half octaves; faculties that enabled her to perform the most difficult passage work.
 In a letter dated 24 March 1770 Leopold Mozart wrote of hearing her perform a C an octave above high C at the Ducal opera of Parma, "I could not believe that she was able to reach C soprano acuto, but my ears convinced me."
 Aldous Huxley also mentioned this event in his novel, "Brave New World", (misspelt as 'Ajugari').
A recommendation letter or letter of recommendation, also known as a letter of reference, reference letter or simply reference, is a document in which the writer assesses the qualities, characteristics, and capabilities of the person being recommended in terms of that individual's ability to perform a particular task or function.
 Letters of recommendation are typically related to employment (such a letter may also be called an employment reference or job reference), admission to institutions of higher education, or scholarship eligibility.
 Recommendation letters are usually specifically requested to be written about someone, and are therefore addressed to a particular requester (such as a new employer, university admissions officer, etc.), although they may also be issued to the person being recommended without specifying an addressee.
During the course of his lifetime (1770–1827), Ludwig van Beethoven enjoyed relationships with many of his musical contemporaries.
 Beethoven was famously difficult to get along with, and the history of his relationships with contemporaries is littered with arguments, misunderstandings, and reconciliations.
 Beethoven had well-known fallings out with his one-time teacher, Joseph Haydn, with the piano virtuoso and composer Johann Nepomuk Hummel, and the German composer Carl Maria von Weber.
Blond Eckbert is an opera by Scottish composer Judith Weir.
 The composer wrote the English-language libretto herself, basing it on the cryptic supernatural short story "Der blonde Eckbert" by the German Romantic writer Ludwig Tieck.
 Weir completed the original two act version of the opera in 1993, making "Blond Eckbert" her third full-length work in the genre.
 Like its predecessors, it was received well by the critics.
 She later produced a one act "pocket" version of the work.
 This uses chamber forces rather than the full orchestra of the two act version and omits the chorus.
 The pocket version receives frequent performances, especially in Germany and Austria, while the full version is available in a recording featuring the original cast.
Cassiber were a German avant-rock group founded in 1982 by German composer and saxophonist Alfred Harth, German composer, music-theatre director and keyboardist Heiner Goebbels, English drummer Chris Cutler from Henry Cow and German guitarist Christoph Anders.
 They recorded five albums, toured extensively across Europe, Asia and North America, and disbanded in 1992.
Friedrich Jeremias Witt (November 8, 1770 – January 3, 1836) was a German composer and cellist.
 He is perhaps best known as the likely author of a Symphony in C major known as the Jena Symphony, once attributed to Ludwig van Beethoven.
Two Little Pieces is a series two small pieces for solo classical guitar, composed by Takashi Yoshimatsu in 1994.
 It was written as a subset of pieces that the composer wrote for classical guitarist Kazuhito Yamashita, originally composed for the piano or harp.
 The composer describes the work as an "envocation of a lonely fish and a white view".
Heinrich Schütz (] ; 18 October [O.S. 8 October] 1585 – 6 November 1672) was a German composer and organist, generally regarded as the most important German composer before Johann Sebastian Bach and often considered to be one of the most important composers of the 17th century.
 He wrote what is traditionally considered to be the first German opera, "Dafne", performed at Torgau in 1627, the music of which has since been lost.