hedonist,
The differences with upgrading a cable WILL be noticeable given what you have right now. However, the cable upgrade is much much more noticeable as you improve your source, interconnect, and amp as well.
Go with the equinox if you are using a bad source and are listening to mostly rock, although its a close decision.
Then again, the Cardas is a pinch more revealing on the top end which might, mind you I say might, do better for rock because there will be a tad more forwardness.
The equinox is supposed to totally extend the bass, much improve the treble, and offers a greatly smooth non fatiguing sound. Good for not so great recordings.
The cardas does a good job of extending and tightening bass, improves treble, and offers a more analytical sound because whilst the equinox refines the treble the cardas does the same and emphasizes it a tad more than the equinox. Both cables are supposed to be pretty darn cool.
I have only heard the cardas, but those who have heard both state these are the differences.
I suggest Jude's review of HD600 replacement cables.
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Oh, and one more thing. Don't trade off the HD600s for a different headphone. Chances are you will hate what you hear and feel disappointed in the end; I find the HD600s to be the most accurate headphone I have listened to. The HD600 + cable upgrade is a real incredible audio deal. I suggest strongly considering it.
Also,
I personally love the rich powerful sound of HD600s for rock music. In a live rock concert I prefer being well behind the mosh pit but close enough to capture the emotion of the recording. I like standing and absorbing the energy of the performance.
Cheers,
Geek