@ditusa However, take everything Acoustic Fields says as an attempt to separate you from your money. I spent $150,000+ on constructing a listening room with the contractor who worked with him on other projects. He never supplied the plans, they were in his head. I had to pay by wire. He put me off several months, then shipped incorrect sized carbon filters, too many acoustic filters boxes, ordered too much acoustiblok rolls and incorrect sized ceiling acoustic filter boxes. After alerting him that the front and back wall box filters were killing the sound (overdamped), he said send me another $30K for front and rear quadradic diffusers. Not they aren't worth it but they were going to be about 15" deep. My room was only 19'6" deep. I would have lost another 2'6" with a room depth of only 17'. Yuck! The exterior of the room was 22' , with 16" thick walls all around. He cost me several months delay in time and materials.
I use two pairs of Shakti Hallographs (2 in the front wall corners are certainly out of the way and two against the mid-walls). I also use 34 Synergistic Research HFTs... The front wall HFTs... serve the function of 180 degree diffusers to a great extent. I would like to add that the front 5'+ high HFT-X diffuser is the most critical to high-mid range balance and the four HFT 2.0s on the side of the speakers are the most critical for the bass vs. mid-treble balance. The other 29 HFTs, not so much but they work together. Hardly anyone mentioned their presence.
I recommend quadradic diffusion if your room permits them. Mine didn't.