This is what I do, but there are a lot of people who are not fans of EQ as you can seeWhy not just apply a little EQ to them? You can even add a bass shelf while you’re at it
The next thing to try is a cable swap. Go with a good all copper cable. Just remember that all cables are not made the same. They all attenuate certain frequencies. You just have find the one that works best with your headphones. You could try a cloth type filter to see if that can tame the highs, but it may alter some other frequencies in the process.Not a fan of EQ unfortunately, I rather do physical tweaks.
I use a Shiit Lokius and it's the easiest way to tweak the sound that I have tried. Everything is done in the analog domain so the sound remains distortion free. Not sure of what else you can do or try with Hifiman headphones.