Davek,
I had the same issues, actually caused by ‘ground-loops’ because of several potentials in equipment.
After using a simple DI unit (e.g. Behringer Ultra DI-Pro with 8 inputs) the noise is completely gone.
In my setup the Yamaha Tyros5 produced the most noise because of the USB connection to PC
Also the PC soundcard output was pretty noisey.
The DI isolates the electrical ground (GND) between in- and output, so ground-loops stop humming.