1.
Sound should be handled as acoustically as possible, without the use of physical filters such as foam or paper, etc.
2.
Remove everything that gets in the way of the sound. If additional structures are required in place for integrity issues, design it so that it helps the sound, as a waveguide.
3.
I very much dislike using glue. Parts should have modular construction as much as possible, and should be able to be easily disassembled for easy maintenance and future longevity.
4.
You can guess how something will sound, but you don’t know actually how it sounds unless you actually try it out. The only way is to actually try it. (Who is insane enough to buy all those HD800s and cut up the drivers to find the best sound? I am…)
5.
Leather pads + Senn = disaster.
6.
All HD800/S should be used without the dust cover!!