hi there, I'm back - and still having noise problems.
Good news is that I finally found out where (at least a part of) the hum came from.
Here's a picture of the now final interior of the case:
I shifted the AC wires from the 30-0-30 torroid a bit and the hum was much less - crazy.
Now there's still some kind of buzzing noise, higher pitched than the usual 50Hz hum.
Apart from that, there's no hum or noise even with open inputs and fully open...