New speaker placement

Look into some treatment on the wall behind the speakers. A nice tapestry might work in your room as the audiophile stuff (look at my system for the ugly stuff) might not fit the nice decor. As for first reflection points, it looks as though the side walls are quite a distance from your listening seat. Nice system BTW.

The treatment ideas in your room diagram look like a good idea. The yellow squares behind the speakers look good as well as the bass traps. As for side wall reflections, it seems as if the walls are far enough away that little would be required but it's fun to experiment.
One thing I can say from my experience is that room treatments do make a difference. The room my system is in now had bare walls when I first put the system in. If you clapped your hand you could hear an echo. As I tried putting panels in different places things changed for the better. I also found out you can go too far by putting up to much absorption.