Complex sentence: But in 1920 (following the establishment of the Bolshevik government), Serge and Natalie Koussevitzky left Russia permanently, and moved the headquarters of Éditions russes to Paris where, in 1921, he established the prestigious "Grands Concerts Symphoniques Koussevitzky".
Split into simpler sentences: But in 1920 (following the establishment of the Bolshevik government), Serge and Natalie Koussevitzky left Russia permanently, and Paris became the headquarters of Éditions russes. In 1921, the conductor established the prestigious "Grands Concerts Symphoniques Koussevitzky".