Hacker News 上看到的，只用一隻螺絲起子，就把 Sennheiser HD 555 升級成 HD 595 的方法：「sennheiser hd 555 to hd 595 mod」。
This page will show you how to turn a $199.95 (Canadian – Suggested Retail) pair of Sennheiser HD 555 headphones into a pair of Sennheiser HD 595‘s that cost $349.95. And all you need is a screwdriver.
兩者的差異只在 HD 555 多了一片泡綿 (foam)，把他拆出來就好了：
Aside from the aesthetic differences, the only physical difference was an additional piece of foam inside the cheaper HD555 headphones, blocking about 50% of the outside-facing vents. Since both the HD 555 and HD 595 are designed to be “open” headphones, reducing the vent with this foam would alter the frequency response slightly. So to save yourself $150, open your HD 555’s up and remove the foam. Done.
作者說是注意的到的差別：
Yes. The actual sound difference is very slight, but it is noticeable.
在 Hacker News 上的討論「Sennheiser HD 555 to HD 595 Mod (mikebeauchamp.com)」裡面有在猜什麼原因，有可能是硬拉產品線，也有可能是將次級品改裝，但看起來兩個機體本身是相同的沒錯...
不過這兩隻都是老機了，看起來現在沒有再繼續生產。