I am at the point where room damping is a real issue.  My listening set up, by necessity, is the "room acoustics from hell" 15' X 55' with the front LR speakers placed facing the 15' and 12' ceiling.  Wood floors no carpet (dogs), All walls covered in drywall.  
The B&W 702 S2's are notoriously bright - in my experience, that is also true. 

Anthony Grimani (YouTube/Audioholics ) recommends 15 - 20% room coverage with alternating damping -diffusion panels.  SonitusUSA is out of stock on the panels that dampen the higher frequencies.  
I need to get the harshness / brightness under control as well.

Here is my Question: Which manufacturers should I be looking at for sound damping and sound diffusion panels?  

Building and improving my gear as I go (current hardware below):
AVR 7.2 / 2 channel music setup:
Denon AVR3700H (105 watts)
Rotel 1590 2 channel power amp connected through the Denon pre outs.  
Bowers & Wilkins 702 S2 (left right) speakers.  
Bowers & Wilkins 802 LCR (center power connection by Denon amp)  
Paradigm Phantom V1 LR Side
Paradigm Atom V1 LR Rear
SVS PB-1000 - YAMAHA YST-SW315 powered subwoofers
Sony 85" TV 

Lumin U1 Streamer
Denafrips Pontus DAC (R2R)
128x128Ag insider logo xs@2xpmreagan2
I had the most luck with GIK and RealTraps. RealTraps much more effective on bass for me than GIK.
Thanks Hilde45 - i’m going through the videos at GIK now. They are a ton of help on design, thanks for that Goldmine of information. I’ll take a look at RealTraps today. 
Hilde45 what’s your experience with these products?  RealTraps it’s considerably more expensive - Is there a performance difference that warrants it?  Denser?
What you may need is acoustic panels on the walls and a nice carpet or rug in the room. Along with that, identify hard surfaces and other areas where reflections are common. A set up that emulates near-field listening is ideal. 

You want to be closer to your speakers. Not too far away.

Thank you Mastering92. Can you recommend acoustic panels that you’ve had experience with?  
I am forced into near field listening given the room dimensions. So that’s not a problem. I get a lot of reflection off the rear walls 2 feet behind me. 
My experience with GIK and Real Traps is as follows:

I have a room with a variety of issues, especially below 200 hz. I managed to get a hold of GIK's corner traps, HF limited bass traps, diffusers.

To be precise:

I had 4 of the bass traps and 2 corner traps.

They were not doing enough to manage the bass peaks.

I decided to get Real Traps Mondo traps. I got two 4 ft tall traps and two 2 ft. tall traps:

I tried these GIK and Real Traps in different combinations, and after extensive -- really extensive -- testing, the Real Traps were much more effective in dealing with lower frequencies.

The company was easy to deal with and represents, I think, an excellent value for acoustic control.

Ethan Winer was very helpful, too. He examined photos of my room and gave me some suggestions as to where to place the traps. He was open to follow up.

@pmreagan2 ,

So are you saying your speakers are setup along the 55' wall and you're firing 15' with your ears only 2' from the back wall? Excessive brightness will primarily require treatment at the first and second reflection points of the tweeter on the side walls.
If that's the case, is rearranging the room an option to try the speakers more centered on the long wall, increasing the time before reflections hit the side walls? Is reorienting the speakers onto the shorter wall an option? Either might help regardless of which treatments you choose.
FWIW, I agree that Realtraps are good stuff. While Ethan has taken lots of cr*p online over other stuff, he has an acoustics forum on the Asylum site that offers much good advice on the questions you raise. 
Hey sbank 
Spencer -  Rearranging the room makes perfect sense but… I can’t do it.  (Never hire an architect with overly contemporary wild ideas.)

So the left side wall is 11’ from left front speaker and brick chimney is 15’ from right front (wall 27’)
Going now to the Asylum site to learn how locate the first and second reflection points. 
It may be I might also need to use additional movable panels for rear and sides. 
that's easy...just get a mirror(floorstanding or with a helper to hold/move it) and a flashlight. Sit in the sweet spot with the flashlight. Move the mirror along each wall and aim the light at the mirror. When the reflection of the flashlight lights up the tweeter on the same side of room, that's the first reflection point. Place your panel where the mirror is. Repeat the process looking for the reflection of the left speaker on the right wall and vice-versa; that's the 2nd reflection point. Place another pair of panels there. Cheers,
I use RealTraps in corners, GIK for ceiling and RPG for diffusion and wall/ceiling corners.  I had it tested by a sound engineer.  Since the pictures were taken, I swapped the diffusion to the back wall and the RPG Bad Panels to the front wall.   

When I bought the acoustical material, GIK and RealTraps didn't have the corner bass traps they do today.  I probably would have used them, but it sounds great the way it is.  Call them for advice.  I used suggestions from all 3 companies!  link: