You should look at diyaudioheaven work on the DT1990 :
back to Beyerdynamic back to measurements home published: Jul-16-2017, updated: Oct-03-2021 NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘iro…
diyaudioheaven.wordpress.com
Frans did a lot of measurements and proposed some solutions for the hideous 8K peak. I already tried the amiron fleece pads, and they work well, even above my expectations, but the peak is just too pronounced to be solved with a foam swap. You can add 2-3mm felt to furtherly reduce highs, but they start impacting the overall level of high frequencies too much, and the 8k peak is not thoroughly tamed either.
I've ordered the filter from frans: it should arrive today, I can give my opinion later on this.