It’s not hard to appreciate what Albert M. Tapper, the AMT in AMT Theater, and his cowriters are trying to accomplish with David, a New Musical (yes, that’s the title): create a brand-new Big Old Musical, with big tunes, big ensemble, big emotions. The project appears to be very close to Tapper’s heart, and, along with collaborators Gary Glickstein (book and lyrics) and Martha Rosenblatt (book), he has played by the rules of traditional musical-theater storytelling. But his team has made several misjudgments.