The jack is just metal contacts. Do you mean, how good is the op-amp driving the headphones? My guess is, it's about the same performance quality as found in the other stages of the 976. It would be a guess though, as I haven't used the 976 with headphones.
Forgive my phrasing, yes the op-amp driving the headphones. I hope it is on par with the rest of the 976...doing a lot of headphone listening late night. Thanks for the reply