You've got to be systematic here - you've confused yourself - it hums with only headphones in, but then you say the buzz changes when you adjust input gain?
So all faders down, all gains down - all channels set to route to the output in the normal way. Phones connected, phones set to say, 75%, main mix to 50%. Two track not set to mix or ctrl room. eq at middle positions.fx to off and pan central. What do you hear - with phones on full, maybe a gentle hiss? Got a hum in this state and with no inputs and no other gear connected, just the power - then it's faulty. if there is no hum, move on.
Step to, turn up a channel fader knob to 75%, and as you bring the gain up on that channel, the hiss should increase - again no noise. If it hums, it's faulty. If you then add the mic, how does that work? Then turn on the phantom - a click is fine, a hum is a fault.
You need to add and enable things systematically, in a sequence you can revert when something happens. It is possible the power supply is dodgy, but that would be quite rare.
report back with how it goes.
EDIT while I was typing you sorted it. You say there is a bit of hum/noise from other mics? Why? They should not hum or be noisy?