Original: The former principal cellist of the New York Philharmonic had been the youngest member of the Los Angeles Philharmonic when it was founded in 1919, and had turned to conducting at the suggestion of Arturo Toscanini.
Simplified: Wallenstein had been the principal cellist of the New York Philharmonic and had then been the youngest member of the Los Angeles Philharmonic when it was founded in 1919.