‘Marco Polo’ is the last of five scripts Michael Imperioli wrote for ‘The Sopranos,’ and he definitely ends his run on a high note. Gavin and Hannibal examine the factors that push Tony B past the point of no return, and Carmela and Tony S into the pool. Plus your hosts point out how this great story gives Gandolfini, Falco and Buscemi ample opportunity to display subtle new dimensions of their characters.