Last summer, Ka spent a Saturday standing outside the doorway to 406 6th Avenue selling vinyl and CD copies of his album The Night’s Gambit. The building’s façade might have seemed inauspicious to passersby, with tattered printouts advertising some sort of massage service through the doorway and up the stairs, but to hip-hop fanatics the Brownsville-based rapper had picked a spot that plugged him directly into the New York underground: The address used to house Fat Beats, not just a legendary record store but a symbol of the city’s grassroots heritage. Ka’s one-man pop-up shop was his humble way of paying tribute to his hometown predecessors. While technology has blurred the physical and geographical barriers between music scenes, Ka remains a thoroughly grounded institution. A worker for the FDNY by day, he has made three solo albums that are accepted by critics as classics, with hushed-yet-steely raps resonating as street-honed scriptures allied to self-production that presents itself as a nuanced soundtrack to New York’s shadowiest enclaves. There is no glossy sheen to Ka’s music — it’s just a brilliant take on bricks-and-mortar rap that’s as sturdy as the city around him.