Due you have an oscilloscope? Check backwards (or forwards if you prefer) from sound output all the way back to the DAC for the hum you are hearing, low frequency oscillations should show up on the scope.
Also check your 1/4" cable is good, verify the output is the same against the head phone jack output hum.
Also check befor and after IC 22 for hum just after the DAC. If you do not have an oscilloscope, construct a probe to check the signal lines and use an external amplifier/mixer and listen to the signal path to find the hum.m Just don't short any pins, which could end up being costly via your time and some parts. I know, iv shorted an $150 unobtainable VCO chip before.