If you are not using REW software, do yourself a favor and take the time to download and learn to use and interpret the data. Use REW to optimize the speaker and primary listening position before adding room treatments. Very likely you will see profound changes in the room frequency response as a consequence of speaker placement (sometimes just 3-4 inches) and the location and height of the primary listening position. After speaker and chair locations are optimized begin to introduce room treatments addressing specific issues revealed by REW measurements. Focus on treating the front wall/side wall corners first, and possibly back corners as well. You really want to improve low frequency decay times with these traps. The range limited GIK soffit traps mentioned by @erik_squires are ideal for this purpose. My advice is to avoid full range soffits. You can easily deaden the room with these traps used full range. It is likely you will hear improved vocal articulation after introducing soffits in the corners. Next most important is mitigating early reflections (< 10-15 milliseconds) revealed by the REW impulse graphs. Treating rear wall, side wall, ceiling, and floor (depending on existing floor covering) is imperative if you want to realize decent imaging. Ceiling first reflections points are also important. Doing this much may get you to the point of diminishing returns.
Do not fail to take advantage of the GIK consulting service. They can help a lot especially with graph interpretation. The purpose of the service is to facilitate sales and not to teach, but they know a lot, and if you persist in asking questions, you will learn much about how to do your room without spending more money than you need to.
BTW, my room is 20x15x8 and I have 3 Soffit traps, two monster traps, two 6 alphas, and 6 home made traps using Owens Corning fiberglass panels. I also have 3 Real Traps Mondo traps. I still consider my room somewhat under treated, but I am now at the point of diminishing returns.
I was an audiophile for 40 years before I understood the importance of room set up and treatment. As a consequence, I wasted a lot of money chasing cables, PCs, and other tweaks, most of which didn't offer 10% of the return on investment I got from room set up and treatment. If you don't address the room, you can't do more than extract a fraction of what decent equipment can deliver.
