Complex sentence: Simandl's "New Method" of playing (over a century old now) is still the keystone of classical double-bass technique, but modern adaptations of, extensions to, and challenges to New Method doctrines are acknowledged.
Split into simpler sentences: Simandl's "New Method" of playing, now over a century old, is still common among classical double bassists. Modern adaptations of, extensions to, and challenges to New Method doctrines are acknowledged.