Roland Guerin keeps good company. A master of six-string bass, he put the bottom in Allen Toussaint’s band during the composer-pianist-producer’s second act as a world-traveling concert artist. Guerin spent 2017, Dr. John’s final year of performing, as the singer-pianist’s bandleader. This fall, Guerin is touring Europe with the virtuoso hard-rock guitarist Paul Gilbert. His prestigious sideman work further includes Marcus Roberts, John Scofield, Mark Whitfield...