In my ignorance, I had completely ignored Allen's beautiful Motecta Trium Vocum because I find that many motets written for equal voices do not translate that well when transposed to fit mixed voices. I had investigated the Matri Divinae Gratiae collection, which was written for SAB, but found it to be prohibitively expensive. Then I stumbled on the Voces8 singing his O Sacrum Convivium down a fourth, and working quite beautifully.
I've used almost the entire collection (there's only 2 we haven't used) in SAB. My choir loves them!! I agree with Luke that you might have to play around with what key works best for your ensemble. We start with the recommended key for mixed voices given in the book and make changes if needed. An example of my schola singing Anima Christi is attached.
Challenges for making the Motetca Trium Vocum pieces work in SAB: 1. You may need to provide a transposed version if your choir members are not comfortable reading music while singing a fourth or fifth down.
2. If your baritone voices cannot reach D above middle C and your sopranos cannot sing middle C (or B) loud enough to hear. I found focusing on chest to head voice transition (and back) for a few rehearsals helped a lot with that... but voice 1 does challenge the Sopranos when sung down a fourth or fifth.
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