Turns out it was a Bruckner motet to be sung within the mass, setting by Josquin. Rarely have I heard Josquin sound better.
I think it was that the Bruckner had opened the singer's ears to the reality of overtones. Its relatively thick voicing makes the overtone action explicit, opening the ears to the implicit overtone action of the earlier, sparer style.
I hadn't known this but Bruckner had been tremendously involved with Josquin's music.