The Duke Ellington Orchestra — led by Mercer Ellington after the maestro’s passing the previous year — brought the legacy of one of American music’s greatest composers to The Unicorn in Ithaca. The orchestra was determined to preserve Ellington’s music exactly as he’d arranged it, and a college-town crowd got an evening with one of jazz history’s most consequential bandleaders, kept alive through his musicians. The big-band jazz tradition, still alive in a small Ithaca club.