-Mass of the Angels (Clark) - overall is nice, but my hope is to get the choir to learn the actual unison Missa De Angelis eventually, so would likely not want to mix the two.
I think in general if you want chant, it is going to be hard to find choir harmonies. Chant was traditionally sung in unison, and any harmonies were played by the organ to support the voice. Most chant settings of the Ordinary (both English and Latin) I have seen are written for unison with organ accompaniment.
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