Quickly made galvanic isolator between the DVB-C box and antenna cable. Removed the 50Hz hum from speakers and signal quality of the broadcast is still around 90% so no problems with picture either.

Very cheap and efficient way to break down the ground loop causing the hum. Only two connectors and two 10nF 250-400V rated capacitors. I made the box from small pieces of PCB and soldered all of them together to form a RF-shield to the inside and a sturdy box. Note that the other connector's body isn't connected to the box frame!

Schematic borrowed from here:
http://www.tkk.fi/Misc/Electronics/circuits/antenna_isolator.html