+ 1 on the advice to change the tube locations (swap them).
Does it make this humming sound regardless of whether you do anything (no music, no volume, etc.)? Is it emanating through the speakers only or do you hear something coming from the unit itself?
It sounds like ground loop hum if it isn't a bad tube.
With regard to Freya build quality, that's a superb piece of gear. I've been inside it and in my humble opinion it is very well made and perhaps even better designed.
