Sir Orfeo is a retelling of the Orpheus legend, which is one of my favorite Greek myths of all time - emphasis on the retelling of the story, because it isn't the same as the original myth. To be fair, ancient Greek myths were so heavily regionalized and re-purposed (depending on who was in charge at the time) that it's hard to find just one to be considered canon, so maybe Sir Orfeo isn't as far off-base as I think it is. Honestly, who knows?
The first thing that popped into my head about Sir Orfeo is that, aside from the Breton influence there, it almost seems to be a convergence of two myths rather than just Orpheus's tale. The abduction of Herodis into the "underworld" has much more in common with the legend of Persephone and Hades than it does in the so-called "standard" Orpheus myth, where [SPOILER]: Herodis (Eurydice) is killed by a snakebite, and Orfeo/Orpheus descends to rescue her.
Thracian Girl Carrying the Head of Orpheus on His Lyre, Gustave Moreau, 1865
This story was a change, because there wasn't really any violence or political intrigue. It wasn't quite... boring, but it also wasn't as bombastic as, say, the Knight of the Cart or the Knight's Tale.
Not present in the story: rippling muscles.
Also not present: pretty much any action at all.
The one thing that irritated me about Sir Orfeo is that there isn't a reason given as to why Herodis is abducted. It just seems like the king of the underworld showed up in a dream and kind of told her he was gonna grab her for no good reason. Am I missing something?
Overall, I really liked this story, if only because I enjoy the Orpheus legend and all its various forms, even Tennessee Williams' Orpheus Descending which follows the legend but... in a really abstract way? What even happened there? Anyway, Sir Orfeo is an interesting take on a very archetypal myth.