[11] As a Canada Council Doctoral Fellow,[12] Duffin enrolled at Stanford University to earn an MA and DMA in Performance Practice of Early Music (1974 and 1977, respectively), working primarily with William P. Mahrt[1] and George Houle.

[9] As director of Case's Historical Performance Practice program (1978–2018),[14] he taught graduate and undergraduate courses; directed the CWRU Collegium Musicum,[15] and the Early Music Singers,[16] and founded the Baroque Orchestra.

He was the first guest director of the Choir of St John's College, Cambridge, in an Evensong service, recounted in his article, "Cracking a Centuries-Old Tradition," in Early Music America's EMAg.

[30] Highlights among his published editions include Forty-five Dufay Chansons from Canonici 213[31] which won the Noah Greenberg Award,[7] A Josquin Anthology: 12 Motets,[32] Richard Davy: St. Matthew Passion,[33] and Gude & Godlie Ballatis.

Several have been covered in the press, including "Calixa Lavallée and the Construction of a National Anthem,"[37] proposing that "O Canada" was assembled from a handful of pre-existing works; it was featured on the front page of Toronto's Globe and Mail.

[46] When his daughter, Selena Simmons-Duffin,[47] joined the staff of All Things Considered at National Public Radio, he wrote and produced historically based theme music ("trixies") for the program.