Dallas Symphony concertmaster Emanuel Borok, who plays a 401-year-old Amati violin, “has heard many of the theories about what makes Amati and Stradivarius violins so special. It’s the wood. It’s the varnish. It’s the cow’s urine in the varnish.” But, he says, “As an artist, I’m not interested in that too much.” What he was interested in was commissioning a concerto to mark his instrument’s four centuries. So he did. (With audio and video.)