First step is to diagnose the problem. Move your seat so that is fairly close to the speaker (say 6ft or so away) and adjust the toe-in so it is about the same as that when you are seated at a normal distance. Now listen to your speakers. By listening in the nearfield, you are listening to your system/speakers, with room effects reduced in prominence. If you can, compare what you hear in this position at home with what you hear in other systems, such as at a dealer's showroom. This will tell you whether it is other parts of the system that are causing the problem vs. the room.
If it is other parts of the system, it is unlikely that room treatments will really solve the problem. You would end up using excessive treatment to kill high frequencies that will not easily die, and the end result will be a lifeless system.
Almost all gross problems are best attacked by addressing placement of the speakers. You need to do a lot of experimenting with room placement, amount of toe-in, height of speaker/and or the amount of back tilt to the speaker. By changing the angle of the drivers to your listening chair, you can dramatically change high frequency balance, hopefully for the better.
As for room acoustics problems that you can ameliorate with treatment, again, start with diagnosing a problem. Have someone walk around the room while talking to listen for problems with the voice sounding unnatural or difficulty in understanding the speaker. Also, in various spots in the room try clapping to determine if you can hear a distinct echo. If you do hear a long and distinct echo, you do have problems that require treatment. An excessively lively room with "slap echo" (sound bouncing back and forth between parallel surfaces), this can be the source of excessive brightness and sibilance. Also look for large flat reflecting surfaces in the path of the sound; one of the worst offenders being a coffee table in front of the listening chair.
As for specific treatment, it is pretty hard to guess what would be the best approach. As a general rule, I would say start with simple things and be wary of overdoing treatment; it is easy to start down that path and becoming obscessed with the changes and then overdoing things). If you have hardwood floors, an area rug in front of the speakers is a big help. If the wall behind the speakers and behind the listening chair is a bare, hard surface, put up a fabric wallcovering of some sort on at least one of those surfaces (by covering one of two parallel surfaces, the back-and-forth bouncing of sound that creates slap echo is reduced). You can also try putting up wallcoverings at the first reflection point on side walls to see if that helps (try blankets as a temporary fix).
For specific, dedicated room treatments, you should consult websites that discuss this matter and contact some of the vendors of products. They are pretty good at advising what is the best approach. I would pick a vendor that makes all kinds of treatment-absorbers, diffusers, combination devices, "velocity-based" and "pressure-based" devices, etc. (GIK Acoustics is an example).
