Original: Under Koussevitzky, the orchestra gave regular radio broadcasts and established its summer home at Tanglewood, where Koussevitzky founded the Berkshire Music Center, which is now the Tanglewood Music Center.
Simplified: With Koussevitzky, the orchestra gave  radio broadcasts and made its summer home at Tanglewood, where Koussevitzky founded the Berkshire Music Center, which is now the Tanglewood Music Center.