So I'm back after trying the Sennheiser HD600 mod on my HE-400s for a few days. Unfortunately for me, this mod actually makes it more uncomfortable to wear as the HD600 headband cushion has 4 foam bumps that causes the weight to be focused onto those four points, while the stock flat headband allows the weight to be distributed evenly on the headband.
I've given up on trying to make other companies headbands work on the HE400. I think the best solution is some sort of DIY project using memory foam + some fabric to make a wrap-around.
this is my first personal DIY attempt: called ghetto mod version 1.0 = using cut up memory foam packaging cushioning for a watch (1in thickness) + twist tie
pictures of ghetto mod version 1.0 (Click to show)
thoughts: Using 1inch thickness memory foam, I went from the tightest setting to the largest setting on the headband, which is fine for me, but for others who already are on larger settings. I think that the perfect memory foam thickness would be around 0.5-0.75 inches. You should be able to trace out the headband to maximize the amount of cushioning to help distribute the weight. You can probably finish off the mod w/ a piece of fabric + velcro to wrap-around & secure the memory foam in place. Legit it feels like you are wearing a cloud on your head. heh!! I love memory foam. I secured it using a twist tie, because I was just trying it out & I wanted it to be easily removable.