As a fellow Atma-Sphere preamp/amp user, I agree with almost all of Rushton's suggestions. I do prefer the Telarc "Carmina" tho'.
Also try "Trav'ling Home" Joel Cohen/Boston Camerata on Erato
"Requiem" (Ockeghem) The Clerkes Group on ASV
"A Land of Pure Delight" (Billings) His Majestie's
Clerkes/Paul Hillier on harmonia mundi
"Ikos" Choir of King's College, Cambridge on EMI
"There Is Sweet Music" Cambridge Singers/Rutter on
Collegium
I envy you hearing these for the first time.
Also try "Trav'ling Home" Joel Cohen/Boston Camerata on Erato
"Requiem" (Ockeghem) The Clerkes Group on ASV
"A Land of Pure Delight" (Billings) His Majestie's
Clerkes/Paul Hillier on harmonia mundi
"Ikos" Choir of King's College, Cambridge on EMI
"There Is Sweet Music" Cambridge Singers/Rutter on
Collegium
I envy you hearing these for the first time.