Need help with acoustic treatment for brightness.

One thing you might try is changing the angle of your speakers and, if they're monitors, the height too. Some say as a rule of thumb that it is preferable to have a slight upward tilt with the tweeter at a height just below your ears at listening position. Like everything to do with speaker placement, your results may vary, so try some small changes and see.

Another Hail Mary attempt is, if your speakers are bi-wirable, make your connection to the woofer side of the terminal pairs.

You have 2 options, which you mentioned. One is absorption. 1 inch Owens Corning 703 fiberglass will begin absorption at around 1200 Hz and by about 4000 Hz have nearly 100% absorption coefficient. So it will do fine at 5000. However, if you don't want to lose the air at the very high frequencies, then diffision may be the key. The low profile skylines or hemi fusors would be in that bandwidth. You can learn more about the different products at www.rpginc.com. They have good specs on all their products, so if you know the bandwidth you need, it's pretty easy to figure out which product will work for you.

Hear's a real cheap and easy solution. Go over to Home Depot and buy a sheet of Hardboard (like pegboard without the holes). Cut it to the size you need (depot can do this for you). Buy some 3/4" poplar and cut it into 1" strips and staple and glue it so outside edge of the poplar is flush with the outside edge of the hardboard. Get some standard fiberglass ceiling tiles and peel off the white face. Use a little construction adhesive to secure it to the hardboard. Sand the outside edges of the poplar with some 150 grit sand paper and make sure to "break" the edges and corners so that when you stretch fabric over the frame it does not tear. Pickup some fabric at the local sewing shop and stretch it over the frame and staple to the back of the hardboard. I made 48 of these for my hometheater for less the $500.00 including the fabric (100 linear yards at $3.00 a yard)! I consulted with Rives audio so that I did not overdamp the room. I actually made some "dummy" panels by flipping them over and covering the harboard side with fabric.

Good luck,

Rives- you are way cool. To offer free advice when you do this professionally is more than generous. I did use your test cd with the Radio Shack spl meter to record the 5000hz. So I guess I gave ya a little business. I suppose I should lean towards the diffusion as my room is pretty dead already. Unfortunately the diffusion option seems likely more expensive than absorption.
Prpixel- thanks for the construction plan. It's nice to know some of these tasks can be solved with some sweat equity.

Holzhauer--thanks for the compliment. I believe education and thus awareness of the issues does ultimately increase business over time, so we are happy to offer these pointers. It's not the same as designing a room, but hopefully it will help.