I thought it sounded like the sound bouncing off of an undamped plastic enclosure. I opened the cups, and indeed, behind the driver is an empty plastic cup. Which explains at least part of this glare. I'm going to see what happens when I damp the cups. Problem is that they are already on the bass-lite side, and damping usually reduces/absorbs bass. So there may not be much that can be done.
Keep me posted on this. I'll be interested in these if that can fix the issue.