Thanks Ethan
It's great having such an experienced person to help me

Of course I already know your site and your videos. And I read and watched them nearly all the way through some time ago.
I've chosen the monitorposition that way round because I've heard in another thread that it is more important to have equal distance to the sidewalls than to the backwall. And because my room is L-shaped, I thought, this would be the best way to start.
So I'll turn around the Mixingposition. Sweetspot is around 1.5m away from the window now.
The bookshelf I thought could act as a sort of diffusor...

maybe I was wrong in scientific terms...
the resonator is a wooden box with Rockwool in it and on the front side, there's a thin plate, that resonates - if I remember correctly - around 60-70 hz. Which was a problem area as I did the testing after your Videoguide with a sinus sweep.
Where would you place those?
Could you please comment the new picture?
Thanks!
Middletom