Hi
Send some more pictures, I would like to see how your listening position looks like. About the curtains, it dosen't matter or would help with your treble problem.
I do belive that you have problems with your bass response and that causes the treble problem. You have to do some room measurements to actually see what the problem is. I can do it for you if it is not far from my home.
Where do you live in Sweden?
Thanks