Skepticism will be there until people actually try the higher quality stuff. It's easy to point to this or that scientific study or say that such and such said cable upgrades can't make a huge difference... But once you TRY it, there's no going back. From firsthand experience, I found out that cables can make a tremendous difference. I have the HD600 and replaced it with an HD650 cord. I felt that it made improvements in some areas, but lost some clarity in the higher frequencies. But there is no doubt in my mind that there WAS a change. The 650 cord gave some weight to the music.
Now, switching the cord to the Equinox was an eye-opener to me. Unless your hearing is completely shot or you've done hypnosis on yourself to somehow dismiss the fact that cables can upgrade your sound, then you will hear a significant difference. You might think it's insane to spend $300 for a cable upgrade... that is until you hear the change. Obviously, everything is relative. What is great for some is minor to others. For me, the Equinox wasn't a minor little tweak in sound. I don't know why cables are so overlooked. They just don't have the 'sexy' factor of getting a beautiful new amp or source. However, before rushing out to get a new amp and a new source, make sure you are getting the most out of your current equipment.
Plugging my 600s with the Equinox, I noticed that the grandfatherly nature of the HD600s with the stock cable was removed. Voices no longer sound repressed... They are now much more engaging. the sound seems faster and more intense, bass is deeper and more distinct, and everything is more visceral. There is simply more impact, as your headphone is filled up with music... It's amazing. If you were wowed by the 650 cord change, definitely look into the Equinox cable upgrade for your 600s, because it spanked the 650 cords as well.
There is a 30 day money back guarantee, so you can get your money back if you think I'm lying about the significance of a cable upgrade. I haven't tried the Cardas. Only the 600, 650 and Equinox. I've heard the Cardas is focused more on treble, while the Equinox is more midrange to bass focused. As I've said before, I haven't tried the Cardas, but from what the Equinox has done for the 600s, the $300 I spent was well worth it... Plus Stephan Audio Art has great service. Read some reviews of the various cables to and find out which area of sound you would like improvement on and purchase one that best fills that need to see for yourself.