A quick Google search found several sites with information on the frequency range for sibilance. They place sibilance somewhere between 4k-7k hz. So, my hearing loss above 15k hz has nothing to do with not hearing it. Maybe I'm just not as sensitive to sibilance as some.
That's odd, I too thought it was in the upper frequencies.
Anyways, I lost the stock felt months ago, but I did put something much like it over the port. I'll try with only 1 layer and then I'll try with just electrical tape as a reflex dost, I'll repost back with results.