i don't have a k50, but i do own a k60 (almost NOS, with a retrofitted 1/4" stereo plug). and they sound exactly like what they are: a sixties can. means emphasized mids and an early roll-off in the treble as well as in the bass. listening to the beatles or the stones totally makes sense though
btw: this is a DIN-plug, not XLR. sort of a predecessor but not compatible.
They sound like an interesting headphone for listening songs from the 60's and maybe 70's, but i never saw that plug before used in Audio, king reminds me of the PS/2 connector that existed on computers until not so long ago, but what devices used to have that DIN connector for Audio? i never saw any vintage stereo around here with that connector, and i must say i saw on eBay a K50 with the ¼" plug, modded for sure.