I don't know their exact release date, but I've heard 70s Stax estats that sounded perfectly hi-fi.

Amps are fundamental to lots of electronics applications, so all of that was worked out pretty early. Hi-fi headphones are more of a niche, and I don't know how big that niche was back then - hi-fi playback at home/stereo sound/etc was all pretty nascent in the 60s. Materials/manufacturing/engineering has also come a very long way since them, especially with the rise of computers and everything that enabled.