Thanks for your very knowledgeable response. Intermodulation seems to be just what i was suspecting - basically a problem of frequency overlap. I am wodering wether this is caused by digital modelling. I only have a fender blackface analog amp, not the right device for testing this. Your argument sounds very convincing that it happens in analog devices as well. I know i can somewhat work around with my playing technique, just as you suggest. I have tried different amps and dirt pedals, even different vst plugins , which made a difference, but helped only so much. Your suggestion concerning ripple sounds interesting - have not looked for that so far and will give it a try.
I have stumbled across an issue, that quite annoys me. Playing with high gain sounds i notice an artificial, howling sound when i do bends on neigboring strings (12th fret or higher). I wonder what is the reason for this and in particular, how can i avoid it. I also get this howl with different daw-plugins. I have attached a short example click (4 takes: (1) dry signal, (2) helix, (3) Amplitube, (4) archetype nolly). Any hints how to deal with this? modelerHowl.mp3