Hey, HD800 modding is far out of my expertise too! I'm just speedballing here and have the data from the measurements and what my ears tell me. Plus the efforts of others in this thread and @Bill-P 's threads. Here's another photo looking at the rear side of the cup - It's a little more open than it probably seemed from that first image. The deepest part of the HD800 cup is about 25mm, the foam is only 12mm so it's not 'full' of foam if that helps to imagine how much foam it is. The speaker stuffing analogy is a bit different - the back of an enclosed speaker plays by a different rulebook than the earside of a headphone. @tommytakis and I are starting to deep dive into sunken baffle headphone design and HD800 mods. Definitely learning as we go but hoping we can pull out some great performance and definitely will share every step of the way. I'm all about rapid iterations, wacky ideas, incremental change, slowly narrowing into things, following the path of least resistance.