It would be interesting to know whether Millennials’ ironic fascination with yacht rock has helped the Doobie Brothers move any more tickets as they tour as a nostalgia act. After all, the Doobies were one of the mid-to-late ’70s radio staples most responsible for making rock music safe for dentists’ waiting rooms. Walking pop-culture punchline Michael McDonald — arguably the man most responsible from transforming the Doobies from a low-calorie boogie outfit into a soft-rock hit machine — doesn’t appear to be in this incarnation of the band. And that may or may not be a selling point to those who still care. Even so, expect one of touring sidemen to attempt McDonald’s signature warble as the three remaining Bros ladle out the pre-chewed hits that keep their gravy train rolling.