Room Treatment

“what application works best for a standard 15x15 listening room with gypsum wallboard walls.”

I recommend you seek consultation from GIK, they provide free consultation. Whatever they recommend, I suggest you treat each area in steps. This will allow you to understand the impact of acoustic panels/diffusors and further evaluate the need to treat your room. It’s easy to overdo absorption and kill the liveliness of a space, especially in a smaller room; hence my recommendation for doing it in steps. Not knowing your room, we are just shooting in dark here. Let’s say you have a conventional room with four walls + ceiling which gives you five areas to address,

Front, Back, Sides, Ceiling and Corners.

Starting with the first reflection points (side walls and ceiling) usually gives the biggest improvement in clarity, while bass traps in corners help smooth out low-frequency response. The back wall can be tricky—some rooms benefit from absorption, while others need diffusion to maintain spaciousness.

Hope this helps!

https://www.amazon.com/Master-Handbook-Acoustics-Seventh-Everest/dp/1260473597

Acoustic panels absorb while diffusers reflect signal (scatter)

Acoustic panels absorb sound waves to reduce echo and reverberation, making a room quieter and more controlled. They’re usually made of foam or fabric-covered insulation and placed on walls or ceilings.

Diffusers, on the other hand, scatter sound waves to create a more natural and balanced sound without making the space feel "dead." They’re typically made of wood or plastic with uneven surfaces.

If you want a controlled sound, go with panels; if you want to keep the room lively while preventing echoes, use diffusers, or better yet, a mix of both.

A lot of good info already provided. The part mentioned about over doing it with absorbers is of the most importance as it happened to me. Bottom line, a combo of bass traps, absorbers, absorbers/diffusers and diffusers in the right combination will get you the optimum results. What that combination is will be specific to your room. It's basically like equipment, cables, fuses, etc... There is not one for all fit. The best I can help anyone with is to take a look at my house of stereo acoustic setup under my details, system.