Eden World Traveller 550 steady low hum

[Throstur]

I am now dealing with a two years old Eden World Traveller 550. I´m not very good at these solid state amps yet, but I am learning. In this case I get a underlying steady hum, not very loud but enough to make the amp unusable. Any idea what to suspect? This thing is packed with op-amps, diodes and transistors and I have measured them all "ok". But what is ok in this solid state world, and what is not?

For example; can a transistor, despite measuring good/working, develop a situation where it produces hum? All tips and ideas on this appreciated.  

[PRR]

Hum comes from the wall. Transistors won't make hum, they put out what you give them, signal AND power.

Filter caps.

Bad grounds.

Note which knobs/jacks affect the hum, that leads you toward input or output sections.

[Throstur]

There is a lot of OpAmps in this amp.  It seems to me that one of them could be the source of this hum. Question is, can they fail in the way that they make hum?

Added 2d of October: The reason for the hum was a faulty OpAmp. Cured.