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.
Main HT <> 976 <> 7200 <> Ultra-X12 <> sapphire ST2 mkII RF/LF <> sapphire SC center
sapphire ST2 mkII SBR/SBL <> sapphire SS SR/SL <> Verizon/DVR <> Vizio E70U-D3 <> Oppo UDP-203 <> Amazon 4k Fire TV