Thanks for this detailed answer, it will help me to find my sound(s)
But belive it or not, i think that what i heard within the first few days based on a hardware problem or a software bug. Because this hissy noise over all distorted or overdriven sounds is gone without changing anything ion the helix.
It's just like after firmware update and factory reset, the unit needed some time to implement all the parameters, specially the speaker simulations. I have really no explanation for this but i'm pretty shure that it were'nt my ears misleading me.
In the first days, all distorted sounds where way brighter and more "fizzy" or farty than everiyhing i heard with se same speakers in the same distance in the same room just a few seconds before on the same reproduction chain. So it sounded a bit like one of the very first modelling units or a cheap transistor amp.
Yesterday, i tried again and - surprise - the noise and fizz was gone - not a bit but completly. The sounds from my helix come now pretty close to samples from the factory presets i know from others. Shure, there still are some differences, but in a range wich can be explained by different guitars, cables and playing techniques.
How can it be possible that a digital devices behaves a bit like a brand new speaker who changes its sound by some hours of use because mechanical components as spider and surround gets more soft by movement. (At the end, the difference was way bigger than between a new and a used speaker)
Perhaps, Line6 implemented this behaviour by software, so a brand new device needs some time to settle the sound? ;)
To compare, i changed for some presets the and+cab models in separate amp and cab with the same parameters. Sounds identical. Then i switched of the cab (it's so great to be able to do all this with the feets while playing) and there was it again, this noisy sound i was used the first days. Just to tell you how big the difference was between before and now. So i rellay think that the cab simulations did'nt grip in the first time.
Then i made a factory reset again. My modified presets were gone but the sound is still ok. So i can not reproduce the effect.
There are sometimes funny effects in a digital world wich should not happen when it comes to processing zeros and ones, but they appear (think on your computer). So perhaps a software bug, perhaps a hardware problem, who knows. But for now, im totally satisfied what comes out of the factory presets and now have a (hopefully stable) base to tweak at my own taste.
So thanks again to all of you trying to help!! I could take some good advices wich will help me to use my helix better in the future.
mikisb's post in Helix: different day - different sound? was marked as the answer