Ian - don't the changes to the housing alter the sound? As compared to the sealed sennheiser plug on the cups, your photos show gaps where the cable enters. Even though these are open headphones, changing ports would presumably impact things.
Secondly, given your know how with cables, is there any chance that you could (or have already) rig(ged) up a test rig to show measured differences in the cables you've made as compared to stock. I've been asking this question on head-fi for years now. Given the availability of freeware like diffmaker, I don't understand why no one has made a diy'd reverse sennheiser adaptor that can be jacked into the input on a sound card. Running computer -> amp -> cable 1, 2, 3, 4, etc. -> diy adaptor -> computer should show differences in FR if these cables really do sound "brighter" or "warmer".