My Sennheiser HD 555 headphones are great -- so great that I've neglected to upgrade them for many years. Thanks to a discovery by clever human being Mike Beauchamp, it turns out you can perform a free upgrade to these headphones and make them equal to their high-end counterparts.
It costs companies money to create several distinct products, so they often create a high-end version and then cripple it for the more basic version. Such is the case with the Sennheiser HD 555s, which can become HD 595s by opening up the headphone casing and removing filters for each speaker. This can save you $150 or more, and is a great reason to buy mid-range instead of high end.
While these specific models have been replaced by others, you can still perform this trick with the newer (and cheaper) HD 558s. While we don't know of non-Sennheiser models with such easy upgrades, this trick potentially works on a few other pairs out there. If you've got a decent pair of headphones, peek inside and you might be able to make them a lot better.
Sennheiser hd 555 to hd 595 mod [Mike Beauchamp via Sebastian (thanks for the tip!)]