Mains hum without ground loop
It was the cable after all. 

It has a loose connection. Unless manipulated, it connects and sound gets through. That is why I didn't notice initally. Even then, I ruled it out as the cause for the hum as it worked most of the time (of course I got a replacement anyway). 

I measured the resistance of the cable:


left channel: 0.9 Ohm
right channel: 0.9 Ohm
ground: 3-200+ Ohm fluctuating, occasionally disconnects completely.


I went a bit CSI on that cable. The shield is twisted together and soldered to the jack by a few strands. It broke of there. The assembly is molded in rubber, the strands stayed in place and still made a lousy connection.

Sorry for the red-herring hunt.

Now that this is settled, what's with those 88 white and black buttons? They aren't labeled. Which one does "Flight of the Bumblebee"?