Bit of a necro post, but I just did basically this same mod to my DT 990 Pro's. I cut out a circle of felt and placed it in the spot shown above. Seems to have drastically reduced the somewhat overly sibilant quality of the DT 990 Pro. Made it much less fatiguing to listen to at volume. Seems to have left the remainder of the spectrum largely untouched. Well worth it, in my estimation.