I tried some eq but after a while I preferred the original sound of hd 650. Im not sure about it but eq might introduce bugs in the audio or degrade audio quality (audio signal gets processed).

mods can be interesting way of changing sound of a headphone but I haven't tested any mods :)

How about the gain setting of ho200 amp ?

"low" gain is optimized for iem. (Accourding for manufacturer)

"medium" gain try this.

"high" gain comes with noice penalty at 300 ohms load according to this website
https://www.audiosciencereview.com/forum/index.php?threads/smsl-ho200-review-headphone-amplifier.27553/

I could not find any info of that amp of yours named hd200. What is the exact name of the amp ?

Have you thought learning eq ? here are some of the basics. Crinacle has this tool to create eq presets for your preferred parametric eq app.

Crinacle tool
https://crinacle.com/graphs/headphones/graphtool/

how to use ?
Select your headphones on the list and click equalize. Make your settings (there is even the autoeq button). Click export to get the settings in a txt file (you need the preamp value from the txt file). Apply settings to your parametric eq app

nickel plated cnc copper mod for HD 6XX. It can be completely reversed if foam disk of the rear driver are not removed.

youtube video about the mod
https://www.youtube.com/watch?v=7ZCnI_fKAAk

Thank you :)

When 2 magnets come close to each other they start to demagnetize and the affect can be permanent... this can cause channel imbalance. I wouldn't risk it.