Permanent 50hz

Is your speaker powered? If so it's probably a bad power supply in it.

You could also try a Hum Buster to see if it is you home wiring.
B

Given your 50hz comment I'm assuming you're outside the US. If the equivalent of 3-prong adapters exists for your local plug configuration, get one and see if the noise goes away. If it does, you have a grounding problem, which you'll have to work on component by component. If the noise stays, it's more likely your amp or an upstream component creating the noise.

Since you have said the hum only occurs with nothing connected to the amp I'm guessing the amp. I'm also assuming the sources are both unplugged from the wall and the interconnects to the amp. Is this a problem that just occurred or is the amp new to you? Have you tried moving the amp and speakers to another room or even a different location like a friends home or audio dealer?