Sh*t, sorry for not getting back to you guys. Well, I had 1) echos 2) smack-the-tongue metallic feedbacks on some walls, and 3) yeah, kinda boomy bass.
1) was fixed with a proper acoustic ceiling, cost 550€
2) kinda but not quite fixed when I moved in armchairs/bookshelf/rugs.
3) fixed with monitor setting, sofa front and center, and heavy drapes over 7cm insulation blocks [edit: in corners].
After
[exturl=http://www.scoopex1988.org/article-my-studio-in-progress.html]these pics[/exturl] I added the furniture mentioned above and put the same 7cm insulation blocks on the doors of the room, I thought they were the source of the "smack the tongue metallic clicks".
With ceiling/furnishing/monitor adjustment it really is quite okay, if I close the door to the hallway/stairs, which is the usual insanely echoing cement walled space...
Seems I set up the speakers correct as well then, phew! Fluke, really

Well, semi-fluke, since I am a rational being with ears. If you get what I mean...
[Edit 2: To complete the response, no sounds were too bright, and I had no problem finding the sweet spot, in fact I got the sweet spot before I made those shelves to fit it, and bolted them onto the desk corners.]