if you would like a little less bright, you should try some sound absorbing (reflection absorbing) on the side walls, between your seating position and the front L/R speakers, and maybe on the ceiling too.
the back of the room should have some diffusing material, like picture frames or better yet, a book case that takes a good part of that wall.
tweeter "brightness" depends a lot on these factors, and if there is not enough sound absorbing/diffusing material, the high frequencies bounce around on the surfaces and reinforce themselves until they hit your ear and make you wince.
