Zac Brown Band became stars in the late 2000s on the strength of the number one country hits "Chicken Fried" and "Toes," songs that seemed like throwbacks to the country-rock of the 1970s. For a while, ZBB continued in that vein, scoring another breezy, beachy hit with "Knee Deep" and underscoring their '70s singer/songwriter heritage with the warm, burnished ballad "Colder Weather," but soon the group proved themselves to be a restless jam band, as eager to crank up the amps as they were to ride a supple groove. This sense of musical adventure led Zac Brown Band to cut a 2014...