I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here, 3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.
After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.
PH