Permanent 50hz

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?

@musicland1

Its often quite normal for an amplifier to pick up noise if nothing is connected to its input. Its also normal for the amplifier to buzz if the preamp is connected but not powered up.

If you want to test to see if the issue is the amp you could try putting shorting plugs on the amps inputs. Then power the amp on and see if the noise is still present.

If there is nothing connected to the speaker except power cord, and we are assuming it's a powered speaker, (no RCA or XLR signal cables) -- if you are still getting 50hz buzz, then I agree with Eric that the power supply is failing (probably a bad capacitor).  The amp/ps board in the speaker likely needs to be replaced.