Thick wool felt is expensive but what is used for effective diffraction control on some speakers (think Avalon's grills). It is really effective for this freq range. Thick wool felt mexican blankets and such are cheaper than buying the raw material much of the time and very good for the presence region (imaging, soundstage, etc) which makes it work great for TV's and such. However, like John illuded to, it will do nothing for bass, but that isn't really needed for this particular application.