This website explains how to turn a Sennheiser HD 555 pair of headphones into a HD 595 pair, which costs $150 more, using just a screwdriver. The only difference between the two headphones is an additional piece of foam in the cheaper version, reducing the vent’s size by 50%, which changes the frequency response’s character. However, removing the foam is simple and will allow you to preserve the expensive headphones’ desirable flatter frequency response curve. This change highlights how manufacturers can cut costs by “crippling” an existing high-end product rather than creating a new one.
http://mikebeauchamp.com/misc/sennheiser-hd-555-to-hd-595-mod/