The headband on my 2-year-old 400se was flaking, and since I wasn't using it anyway due to the headband being a torture device, I decided to mod it lol.
I removed the old headband (irreversible) and ziptied hd600 nuggets on the metal band. The hd600 headband fits like a glove (same width as the 400se metal band). It looks kinda funny but it's better than not being used lol.
It works surprisingly well, but I might try other headbands later on since this aliexpress hd600 headband is also pretty stiff.
Nicely done, it looks weird but good. You can get suspension headbands on Ali Express that fit Hifiman for $30+ if you want something more permanent.