Music City Roots usually has an upstart act on the bill every Wednesday, but that’s not exactly the case with this bunch. Sure, the Boxcars won the IBMA’s Emerging Artist award this year, but they’re also a bona fide modern traditional bluegrass super-group. Daphne Willis and Matraca Berg — each in her own way — work at the intersection of smarts, heart and accessibility. Willis has a pair of sanded-smooth singer-songwriter soul-pop albums for Vanguard under her belt, and Berg — the Hall of Fame songwriter — writes sumptuously tuneful narratives that beg to be heard. Bobby Bare Jr. and Malcolm Holcombe, on the other hand, have long histories of pushing the artistic envelope, Bare as a purveyor of deliciously skewed, completely unhinged roots-rock and Holcombe as a sharp and surreal country bluesman. Now, that’s a lineup.