I took a gamble and purchased an aftermarket Fidelio X1 replacement cable off eBay from China (
link). I'm fond of the way the X1 cable looks, so I was looking for a cable that looks nearly identical but would have lower impedance than the stock cable.
Shipping from China took 7 days to get delivered (USA). The cable appears to be an identical gauge as the stock cable. It is sleeved, but not with nylon so it doesn't feel or look as nice as the stock cable.
Here's the business end of the cable. The aftermarket cable has the same spring mechanism as the stock cable to prevent stress wear, though the spring is more tightly wound. Most of all, I'm very happy to now have a twist off 6.35mm instead of the bulky 3.5mm adapter that comes with the stock cable.
The headphone end of the cable is silver. It's a slight hue off from the silver on the X1, but a good enough match.
Several lengths of cable are available. I opted for the largest 3m cable. As you can see below, the impedance is much better than the stock cable. I don't have golden ears, but just a quick A/B test and I could tell the difference in sound.
So there you have it. If you are looking for a cheap replacement cable for your X1 that is similar in looks to the stock cable, this one might be a good option.