I cut the grill out.
Long story short, I used to have an HD595, that also had this treble spike. I did quite a few mods to make it sound more open and get rid of the spike. I literally gutted that headset, so that the only thing left was part of the driver, and what held it inside. I took everything out. My goal was to remove anything that the sound could bounce off of, or distort it in any way. The end result was a headset that to me sounded better than a stock HD600 or HD650. I could still tell what was missing in the sound, but for what I paid I could complain. I did a ton of research into how sound travels, reflects, distorts, and anything that really had anything to do with audio. Then after a few years, due to how I had modded it, the driver started to warp a little, into the slightest oval shape. This caused the driver to buzz at certain frequencies. It was such a small error that I could find a way to fix it, and the headset had already been discontinued. I was at a point where I needed ear-buds more than a headset due to the traveling I started, So I got the Klipsch X10. They sounded better than the stock HD595 by far, but not as good as my modded HD595. It worked great in the end as I was using my phone as my output. First a Samsung S3, and then an S4.
I'm now at a point where I still use the X10 when walking to work, but at home I wanted something a little better. I finally caved in and got the HD700. I really want to try the HD800 just to see how good everyone says it is. The HD700 sounds great but it wasn't as great as when I got my HD595 and first stepped into the audiophile world. I have spent so much time listing to music just to enjoy where it can take me.
In regards to getting rid of the protective grill, It completely fixed the spike in the treble, but the sound has changed significantly. I'm not sure how to describe it to someone who listens to a reference design, and what other people are looking for. I'm happy with the way it sounds now, but it's not up to me to tell them if its better or not and for what reason or another.
Here is a current picture of what I have done