The Infamous Stringdusters have been known to stretch the definition of bluegrass to include songs by The Grateful Dead and The Police. But for Let It Go the Nashville quintet headed to White Star Sound, a secluded studio on the rural outskirts of Charlottesville, Va., to explore where their sound would take them with naturalistic self-production. Like most highly respected bluegrass bands, they play impeccably and precisely, crafting vocal harmonies that are as careful and neat as their instrumental prowess. “Middlefork” champions this instrumental ability, while “By My Side,” “Summercamp,” “Peace of Mind,” and the gorgeous ballad “Rainbows” illustrate their abilities to write sweetly crafted tunes that make smart use of upright bass, banjo, fiddle, guitar, and dobro. They clearly use bluegrass only as a starting point. These songs stray to acoustic rock and folk terrain, with touches of country and gospel. The title track sticks to acoustic guitar and fiddle to complement the group’s stellar multi-part harmonies.