Alright guys - thank you so much for putting your effort into figuring out the problems plus all of the things I forgot to mention in the original post!
Took me a while to get behind all the knowledge you dropped in this thread but it seems like it´s really just a bad coincidence.
The D.I. volume is actually all good in all of my presets except that SynthBlink one with the mentioned split and the Industrial Fuzz.
I started messing around with the gate and the In-Z settings in the input block as I was suspecting this to operate before the signal is send to USB channel 5. I just wasn`t persistant enough and took the easy way by posting a forum thread ^^ I learned a lot from your short discussion and the Excel chart - big thanks for that! :)
However, it´s still not a hundred percent clear how to fix this Preset without working a way around the split and the fuzz which does, of course, unwantedly affect the sound.
It´s not like super important - there will probably be no need to re-amp that synth/guitar type of sound anyway.
But just in case someone is able to reveal the 'magic' behind it - I would be interested :D