Thank you akdave,
Update to the "set up": Sub is now on the shelve of the desk (so it doesnt take away place actually). The coverage of the sub is now pretty perfect, the characteristics of the sub got back !
The sub is so much higher (1,5m) and the floor is now vibrating much less. Even my sister has now a better work condition, but the staircase ... well, the wall to the staircase is thicker (20-30cm, half ytong, half brick I guessed).
I am using now 3.1 until I have my cork and it isnt that bad, but 5.1 improves the overall experience in games for a little bit more (even in my small room). I will stick foam on my door, so my mum wont hear me then, because 2 rooms away, she sleeps. The idea with the old gymnastic mats is a great one (no doubt) but I will stick now to my cork I guess :3 It´s good to have people here, even I feeled now the "nostalgic feel" you wrote down in the comments. I had to laugh actually, I am so happy that I landed in a great community ! I can´t thank you enough !
I hope I have a good taste in gear, otherwise my life would have been until now a complete lie I am never searching for the most expensive affordable, I always searched for realistic solutions for a long time. So yeah, Hannes Hauptmann my name, I am now a little bit of a "audioholic"
I will enjoy my journey I guess, the way is the goal !