if a add bass traps is they suppose to be in the sam corner as the sub or opposite or both? and behind the speaker what is best diffusor or absorber?

You can put bass traps in all the corners. Diffusion is also helpful in the corners. TubeTraps can give you both. A stack of TubeTraps between the speakers on the back wall or moved forward a bit is also helpful. It can solidify the center image and improve imaging overall by blocking some of the sound that would normally travel from the speaker to the opposite corner and opposite sidewall.

Here's a short article about the use of a center trap for improved imaging:

https://www.acousticsciences.com/new...ipulation-2-2/