The Ave verum K618 keeps getting more interesting. Example three, version 1:


And version 2:


I believe either would work. For a choir tending to go flat, the first might be preferable, and the basses must take responsibility in the second measure for the shift in tonal center to a comma-high F. Three bars later the basses must return their F from a local tonic to a seventh on “un-“.

In the second, the altos take responsibility for creating the dominant of the comma-low F tonal center. Two and a half bars later they must mentally shift that F on “-tum” from momentary tonic to future seventh and descend an untunable minor third from seventh to fifth: in other words, must listen to the other voices rather than finding the minor third in their voice : it would be too large, and will cause the ensemble pitch to drop.

Is it too much to ask of a director that she think this through and assign one or the other?

Intonation symbols identified in an earlier post: