He represents a number of the world's leading classical music artists, including the Polish composer and conductor Krzyszt Penderecki, the pianist Helene Grimaud and violinist Joshua Bell. He also represented the Czech conductor Jiri Belohlavek until his death earlier this year.
Staging Dream of the Red Chamber in China though was his biggest success so far.
The production of the opera, composed by Chinese-American Bright Sheng and with a libretto by Sheng and Chinese-American scriptwriter David Henry Hwang, is not Chinese but that of the San Francisco Opera.
"It wasn't produced for the China market but by San Francisco Opera, which has a strong commissioning program and tends to commission something by a Chinese artist every five years or so.
"Its board of directors really hoped it would come to China and we were one of two or three agencies who bid on the project."
There were six performances in total, two each in Beijing, Wuhan and Changsha. The first performance at the last venue marked the opening of the Changsha Meixi Lake International Culture and Arts Center designed by the late British architect Zaha Hadid.
"In Changsha the acoustics were brilliant and the look is completely different and wonderful. The center looks like orchid blossoms when seen from above," he says.
Although it was a US production, many of the artists had to be hired in China.
"The San Francisco orchestra was in season and they don't have a double orchestra or chorus like, say, Vienna (Philharmonic and State Opera). So we auditioned with the composer and hired the best young Chinese singers. They had all trained in London or worked in Berlin," he says.
All the performances were a sell-out with Wuhan's being broadcast on local television; and now Armstrong is looking to stage the opera in Shanghai next year before heading for Europe the following year.
"If we could do between six and 10 centers that would be unbelievable. We would like to do the music festivals in Berlin and Amsterdam, the Proms in London, although the Royal Albert Hall (the main Proms venue) is not an opera house, and maybe the Edinburgh Festival and the Proms in Warsaw," he says.
