Complex sentence: Mahler's habit was to subject the orchestration of every new orchestral work to detailed revision over several years; though the musical material itself was hardly ever changed, the complex instrumental' clothing' would be altered and refined in the light of experience gained in performance.
Split into simpler sentences: Mahler's habit was to subject the orchestration of every new orchestral work to detailed revision over several years. Though the musical material itself was hardly ever changed, the complex instrumental' clothing' would be altered and refined in the light of experience gained in performance.