Front or Back wall diffusers

A room of your size will require somewhere between 100 and 140 ft2 of 2" acoustical panels between the 4 walls and ceiling, plus 2-4 Bass traps.

Without knowing more about OP's room -- wall materials, rugs, furniture (especially couches, etc.), windows, etc. I'm not sure where in this estimate his room falls, indeed if it falls within this range at all.

No disagreement that doing the math is important, but the math can only be done if as much as possible about room contents and materials is known, first.

GIK and companies like that need to sell product. Yes they analyze your room but they are going to try to sell you as much as they can too. Most of the people on 

Oops! Most of the people on this forum have the means to consult a professional acoustician and I am surprised most don’t. Jeff at HDacoustics is very good and will analyze your room and tell you what is needed and where to put it. He has vendors he works with as far as panels go but he doesn’t push you to purchase. You can use his design and go from there if you want. Good luck! 

That's really a loaded question. There are endless variations, opinions, rules and suggestions to treating a room. I used all the above when treating my room but ultimately it came down to what sounded good to my ears and not so much following the rules to the letter. Start small and focus on treating your room to improve sound quality, not to look cool. 

Just load up the room with a couple dozen 5’ - 6’ artificial Ficus trees. They are cheap and very effective. Use along the walls, between the speakers and alongside and behind the listening position.