One of the amusements of my simple life is reading old harmony books.  One of the wildest is  Aural Harmony revised by Franklin W Robinson (Hill-Coleman, 1936).    Under the heading of “The VII7 Chord of the Major Mode”: 

“If we transform the VII triad to make it dynamic by adding to its structure a minor 7th interval, we do not change the quality of the VII triad, but we make it more vitally intense and dynamic.  This addition of the minor 7th to the structure of the VII triad makes the chord a symbol of fear, apprehension, and foreboding.   When a chord possessing the quality of sombre unrelationship becomes dynamic, a chord of sinister quality results.   It is very natural that a chord of such quality should be found on the root-tone VII of the major scale.   This result again confirms the belief that the more remote a root-tone is from the key centre the less valuable to the key is the chord built upon it.   “

:”With the III7 we have really reached the end of the related diatonic chords of the major mode.   I believe it to be a fallacy to include the VII7 chord in the class of related chords of the major mode.  It is inconceivable to me that a chord structure implying the quality of sombre unrelationship can in any way serve the major mode. “

“… It is therefore faulty reasoning to consider the VII7 chord an incomplete V7 chord, i.e., without its root-tone, and with the diatonic 6th scale step of the scale added to the structure.” 

I have to disagree with Professor Robinson: I believe a) that a viio7 is in fact an incomplete V9 chord and b) that the 6th diatonic scale step helps in the ‘return’ down the circle of fifths, toward the flat side, emphasizing this role of the dominant seventh.   And this gives a clue how to tune it:   just as the seventh in a dominant seventh is tuned like the fourth scale step, with a Pythagorean third, not directly tunable, between fifth and seventh, so the 9th in the dominant 9th is the low-tuned sixth scale step, NOT a perfect fifth above the fifth of the chord (or the second scale step).   So the seventh has a particular tuning, but is dissonant to the root, the third, and the fifth, and the ninth has a particular tuning, dissonant to the root, third, and the fifth, but consonant to the seventh.  Thus, presumably both should be prepared, but may be prepared together. 

But Robinson may have a point about its character: I began thinking about how to tune it because I was using it in the oratorio, Moses at the Jordan River, at a point where the people of Israel are complaining about their life in the desert: “Would that we had died”.