I can't speak on the J3 but I've read the claims. It's also firmware related too, they may have the same DAC but a lot of the sound will come down to the firmware version they're running and intended sound sig. I would love to know if the J3 does indeed sound the same, but I can't see it, even after reading the claims on iAudiophile. They even measure similar.
I can tell you now though, the S9 and C2 do not sound the same with the S9 being ahead in SQ.