The conventional velour pads fitted to beyerdynamic DT880 have an array of holes on the inner part. I have obtained some handmade leather pads from ebay  - they fit well and to my ears sound OK. But Iǘe heard it clamed that those original holes are there for the purpose of controlling sound...

I am preparing to build an Oatley K272c headphone amp, it has a coupe of tiny valves in there and can be battery powered so is sort-of portable ...I welcome ideas about housing it in a cool way , something with a "steampunk" theme maybe?