I would have to desolder the power-jack and don't have acces to my soldering tools right now
That is a bad thing. Why'd you put em away when it didn't work in the first place? Hmm. Buy another $10 iron and keep it in your backpack!
It could be lots of things. Do the ICs get warm, at all? Any different than ambient air temp? (unless you're sitting on a radiator or working in the snow, that is)
If you futz with the jacks, both input and output, can you change the distortion? Do you use an input cap? is there a resistor to virtual ground after it? What value load share resistors did you go with? What is your source? Can you use a source with it's own attenuator like a portable CDP and see if less input clears it up?
Usually you simplify; work methodically forward or backwards until you find the spot(s) of trouble. This requires jumpers and (de)soldering.