Before jumping into solving the problem lets define it first.
Are you planning on facing the 12ft or the 16ft wall? (IME that's a small room for HT, unless it's meant for a single listener. Can't balance side/rear speakers for more than one listener at those distance restraints.)
What is the average ceiling height? Is it a gabled ceiling (center ridge), shed ceiling (edge peak), or hip ceiling (center high point)?
Describe window/door/opening sizes/locations. A floor plan would be very helpful.
How many speakers/subs are planned?
Have you measured the room yet (to know what if any problem exist)? Why are you concentrating on the front wall? Recommend reading Floyd Toole's "Sound Reproduction" 3rd edition to learn about speaker/room acoustic interactions.
Diffusion is only effective down to roughly 1100 Hz (for 7 inch deep diffusers). Most commercial diffusors are more decorative than acoustically effective. To be effective the material must be acoustically reflective (like painted wood), use a mathematically varied depth of individual elements, and ideally be oriented vertically and horizontally. A good design can be found here:
http://www.pmerecords.com/Diffusor.cfmAbsorption can be effective much lower, but again most are minimally effective (wrong material, shape). GIK offers effective, affordable panels.
