4 channel balanced Beta22:
relay stepped attenuator (64 steps @ 1dB) controlled by an ATTINY26 MCU, used a linear pot. with the internal ADC as control.
Some labeling work left to do, I'm thinking of using Dymo tape.
There IS a bit of hum when using with sensitive IEMs, I might add a bit of shielding around the transformer, see if that helps.
My first suggestion would be to clean up your wiring, a good rule of thumb is keeping wires, carrying voltages with a peak to peak difference of 10 or more, at least 1 foot (30cm) apart. A quick google search for Hong Kong wall voltage is 220VAC, your audio inputs, which we'll err on the safe side, at 1.5v is a difference of 146, even your outputs, run wide open at 8x gain (12v) would be a difference of 18. So all that said, if you have the ability do a reading off the output to see what frequency the hum is, once again google says your wall is putting out 220VAC at 50Hz, so if you're reading a 50Hz signal, then I'd say that the wires bringing in your mains voltage are causing your trouble.
If that's not the trouble, try disconnecting the wires that are used for gain switching, even though they're shielded, they might be picking up some noise, or as stated elsewhere in this thread, the issue with C2-C5 might be the culprit.
If you have a spare case, I would move the transformer and power supply board to it and see if that clears up any of your issues. I have mine built in a two cases and I've noticed that if I place the two close together, I'll start to hear some hum in my headphones. The solution? Keep them far apart and never think about it again.
Now, just because I don't see it, doesn't mean it isn't there, but, do you have the ground loop breaker installed (might not be necessary with your set up, but I don't know you're full topology)?
Ground loop breakerFor option 3 above, the ground loop breaker is a 10Ω 5W resistor in parallel with a 0.1µF capacitor rated at least 250VAC. For safety this capacitor should be rated for class X or Y (good for across-the-line use) with flame retardant casing. The ground loop breaker should be connected between the signal ground and the chassis (which is in turn connected to AC earth ground via the IEC power entry receptacle). Mount the resistor and capacitor in a secure manner so that it will not come loose and come into contact with other circuitry. A good way to do this is to use a terminal strip.
You say you're going to do some tests, for fairness sake, you should get yours operating correctly, and without modifications, first.