You probably saw P.O.S. at 2009’s Warped Tour, engulfed by the crowd as he rapped on top of a beat-up instrument case. Forsaking the stage for the intimacy of the pit is an old punk-rock habit — and an instinctual move for this Minneapolis rapper who grew up on vintage punk acts such as Minor Threat and Black Flag. Stefon Alexander’s stage name is a fluid acronym that has stood for Pissed Off Stef (a remnant of his early punk days) and Product of Society, to name two. Mashing rock and metal with hip-hop beats, P.O.S. mixes genres while focusing his lyrics on social critique and brotherhood. As the title of last year’s release, Never Better, suggests, P.O.S. has honed his skills, boasting a road-hardened live show and a crystallized sense of purpose.