Chicago jazz hero Kurt Elling and CSO bass Robert Kassinger are longtime friends and collaborators. They might not share the Orchestra Hall stage on the same nights, but that doesn’t stop them from making music together. “Symphony Rob” joined the Chicago Symphony Orchestra in 1993, just a few years before Kurt released his debut album, “Close Your Eyes.” Here’s their recent collaboration on a song from that record, “Those Clouds Are Heavy, You Dig?”