Room acoustics: diffusor or absorber?

Diffusers would work best - absorbers would just deaden your room. However since your chair is against the wall I think you would not really gain by treating the back wall at all. The further out into the room you put your chair the more you want to diffuse the reflections off this wall.

I have a setup similar to yours and I use three RPG ProFoam Level 1 difussors mounted on foam board. Works great. You can also make your own RPG Skylines clones cheaply.

I made my own skyline diffusers.A 48"x48" for 23.00.They are not to spec but......neither am I. It does involve purchasing a cutting tool for about 30.00 . I'll send you pics and advice if you want.When it warms up I'll take them outside and spray them the color of my walls with a wagner sprayer and some flat gray waterbase paint.The difference in the plain walls and the skylined walls is unbelievable.In my mind room acoustics are the last frontier.Good luck.

Thank you guys!
I will make the diffusor then.

Check a thread titled "Diagonal room placement." Rhyno shows how to make the clones.