I have been having some offline discussion with WoH regarding mods to the YHD1 and I thought I would share my "secret sauce" for these ( and other) orthos.
We all know ( as the original manufacturers understood ) that closed cell foam has some amazing properties that make it suitable for modding drivers. Several years ago, RhythmDevils stumbled on a particularly good foam that was consistent across a series of headphones. It seems a little odd but the image below is what I have been using ( along with felts etc ) in many different mods but with particular reference to the YHD1 = it is the only thing you need to make them sing.
It has to be the unlined for sensitive skin type = this is heat press molded and changes the properties of the foam to be perfect for damping.
I use 1/2 to 2/3 width outer ring as the pads and cut the inner disc in 1/2 to create 2 "roundish" pads to go behind the driver.
Here are my modded YHD1
..dB