Room acoustics basics

I called GIK Acoustics for help but  they are selling their products 

Couldn't believe how much it help the sound from a big ball of sound to an amazing soundstage with such focus and bass control 

you can check out my room pics in systems

Jim Smith wrote “Get Better Sound”

which includes optimal speaker placement and room acoustics basics

 Check it out 

Good luck Willy-T

After I purchased speaker cables from Morrow Audio I received an offer to sign up for their Tweaks Newsletter. I received the first one and it was pretty basic ( what the OP wanted ) yet informative. It quickly and informatively described treating the wall behind the speakers and the wall behind the listener with easy to obtain and affordable materials. You don't have to make a purchase from them to subscribe to the newsletter. Just go to:

I used the book Master Handbook of Acoustics. Loads of great information.

agree with @willy-t  on Jim Smith's "Get Better Sound".  Much of his very understandable non-STEMmy book focuses on room acoustics and how the room is the #1 component in your system.  Worth the purchase--helped me a great deal.