For more than 50 years, their distinctive harmonies and stellar musicianship helped define the sound of country-rock. Still led by the band co-founder and multiinstrumentalist who wrote and sang their biggest hits, they are a dedicated unit of four formidable songwriters, vocalists, arrangers and players. And rather than rest on any of these considerable laurels, they instead hit stages coast-to-coast armed with the kind of classic songs and instrumental chops that puts performers half their age to shame.