I have a confession to make. I'm absolutely terrible with a soldering iron. When my 600 Ohm DT-990s cable went bad, I was able to fix it and knew I was more likely to damage my headphones than fix them. It's been a long-standing complaint that the high-end Beyerdynamic design has a hardwired cable, whereas many other high-end makes have detachable cables.
I tried looking locally and called a few audio shops, but none did repairs. Then I discovered a guy on Etsy who specialized in a detachable cable modification. I forgot about it until December when I was on Etsy, doing a bulk of my Christmas shopping, and decided to take the $75 plunge as it'd not only fix but improve my favorite pair of headphones I own. I could write quite a bit about my thoughts about the DT-990s, but the fact is if you're interested in this, you already own a pair.
I also did a video version of this blog post.
I have to say, I'm genuinely impressed with the headphones. Matt of demevalos, was responsive, even during the holidays when surely he had better things to be doing, and the turn around wasn't bad considering the massive log jam that is the USPS among holiday rush and defunding.
As far as changes to the headphone's quality, there's no introduced noise or degradation to the signal, granted my headphones have been out of commission for months so. They're as good as I remember. They're the same crisp, slightly overly bright, articulate, tightly coiled, deeply responsive bass.
Demevalos also does balanced cables for the DT-770s, DT-880s, and DT-990s and the 1770s/1990s. Besides being spending and correcting a design failure on Beyerdynamic's end, I have to say I highly recommend this mod.