The Singers Unlimited changed everything in 1971.
4 voices, a 2″ 24-track reel-to-reel, and the brilliance of Gene Puerling combined to create A Capella, the first of their 15 brilliant albums as a vocal group.
Perfection. Warm, harmonically thrilling perfection.
I shared this recording with students this week; it holds up so brilliantly, even in the era of Melodyne and digital overdubbing.