OK - since I started that thread: A BIG THANK YOU!!!
To all of you guys - really! I appreciate your help, and in this case, your patience! Can't explain where I had my head last night - couldn't sum one and one. Had I kept my thoughts a bit more together, I would have seen the difference of DB25 / TDIF myself - since it's been weeks now, I came across different cables, and gear!
I must point out once more - I'm not a pro (you might have already guessed that); being a amateur is hard though, but my own "studio" all long cherished wish. So to plan, build, and gather info on each topic at the same time is quiet hard. But I promised myself I'd be through with this project before I'm 40 (so there's just three more years to go). And recently I got father for the first time. So to summarize: thank you very much, and thanks to everyone who has enriched this thread by posting solutions, answers, questions, and info on any kind!