I'm having the same problem, maybe even worst. Not sure if it's normal or not. When I turn mine up past 12:00m, like around 1:00 it makes a 60 cycle type him/buzz. Nothing I do stops it. Touching strings, nothing. If I turn the tone on the bass down, it gets quieter, but it's still there.
I've tried this on two different basses that don't have any grounding issues.
Under the 12:00 mark, I don't hear any noise.
Basses are cool.