How can I get rid of a 4khz-8khz room spike?

Using different speakers in various positions and measuring room response with a RS SPL meter, I get a consistant 10 db spike over the 4-8 khz range in my small 11x13 listening room. I have just installed a full complement of room tunes and now must decide on other acoustic treatments and/or tweaks to incorporate. I have yet to place anything other than a small loveseat and leather chair in the room, so the walls are barren. What would best attenuate this 4-8khz bump - wool tapestries, rigid fiberglass, diffusion panels, other???