Thank you so much for getting back to me, Martin. I really appreciate it.
I've tried using parametric eq to pull down some level in this area, but I find that the effect of the reflections is different than just an eq boost.
I'm noticing this effect with picked acoustic guitar. I can actually hear a very subtle, very short delay effect that eq doesn't address. It's almost like a ringing of the picked notes above 5k. The eq can turn down the delays along with the source signal, but the volume ratio between them is unchanged.
The foam does a really good job of clearing it up. Maybe it's something I will install when I'm using the mic and remove when I'm done. Do you think the moisture issue would still be a problem if there is a small gap in the foam ring to allow air to get under it?