Parsifal is the Wagner opera which the Kellers go to see in the evening in question. We learn the title when we see a black poodle peeing on the playbill, and the opera is mentioned for the third time at the end of the night when the prostitute is leaning out the window and watching the crowds exit. Again, three mentions in a short story cannot be a coincidence.
Parsifal is a sort of grail-quest story. Lots of religious references, lots of obstacles in the hero’s way, and a big old redemptive ending. We’re tipped off to some possible connections with our story when we see the absurdly epic language used in "Chorb." Chorb is on a "quest," he’s trying to pass a "test," at one point his eye is "burning with a mad flame" – you get the picture. Just like with Orpheus, though, we need to decide whether or not Chorb is a parallel to Parsifal (the opera's hero) or a parody of him. Yes, Chorb is in some ways an epic hero, but he’s also a little goofy. Just look at his moment of victory: "He moved onto the green couch, and sat there, clasping his hairy shins and with a meaningless smile contemplating the harlot." Chorb is also unable to communicate (he can’t tell the Kellers the truth about their daughter’s death, nor can he speak at all at the end of the story) and unconscious of what he’s doing (he’s sleeping and has to wake up suddenly for the story’s climax). So take it as you will.