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.
The only written review says the headband is "well padded and hard" so I guess it is pretty stiff. It should be less of an issue on sennheiser headphones since they're pretty light.
Personally, I prefer the 650 headband over the 600 nuggets. I just got the 600 band for free when I bought my 6XX so I had one lying around.