Regarding String Quartet No. 13 + Grosse Fuge
Prof Greenberg argued that the String Quartet No. 13 is a series of disconnected movements united by the Grosse Fugue at the end which contains motives and connection with every single previous movements. Hence he believes it absolutely should be the closing movement.
He believes the grosse fugue plays a similar role to the opening fugue in String quartet No. 14 where the first movement connects every subsequent movement of that quartet.