Speaker placement advise

Hi,

I've recently purchased the Buchardt A500 and placed them as shown in the 2/3d illustrations (to the left and right of the TV screen). The sound was rarely good enough, suspecting all sorts of returning/stading frequencies, etc., and I would like to understand what can I do to improve it. Can you advise what I can do (reasonable changes) to improve the sound, add reflective foam, or any other suggestions?

Also, me and my partner usually sit in a different positions on the couch (where the red parts are). What will be the best angle to place the speakers so both of us will enjoy as much as possible from the best sound experience?

Drawings:

Thanks in advance!