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.
Need help with acoustic treatment for brightness.
I have switched back to my Forte Model 40 preamp (Pass designed). I like it for it's fantastic detail. However it has given my system a mild brightness issue at around 5000Hz. I have read that absorptive material at the first reflection point can help tame this. In my case, the primary first reflection point is on the ceiling so the solution is not as simple as hanging a rug on the wall. My budget is about $100 so DIY is most likely. John Marks of Stereophile uses RPG Skyline diffusors on the ceiling for his speakers. Could diffusion tame brightness? If not, Is absorption the key? I've used a lot of interconnects and that doesn't solve it.