I run a pair of HD600s through an Antique Sound Labs DT OTL Mk III headphone amp (tube). I like what I read about the Equinox, but they are listed as "warmer" than the Cardas. Most reviews I see rate the Equinox higher overall. Would the less "warm" Cardas cable be a better match for a tube amp that will already add a bit of warmth? Prices are comparable, so it just comes down to a matter of better match. I don't mind the waiting period on the Equinox cables (if they are a better choice). Unfortunately buying both and doing an a/b test isn't an option.
I wonder if there might be some headphone fanatics in your area that might be able to help you out with auditioning the 2 cables side by side. The community over at Head-Fi can be very helpful and generous.
I've listened to the Cardas cable and feel it's a little bright and not ideal for long listening sessions. I ended up outfitting both my 650s and 600s with Equinox cables. I've briefly listened a variety of headphone systems with my 650/Equinox and I've come away feeling I made the right choice. At home, I use solid state headphone amps.
If possible, you should try to hear both cables with your gear. Only you will know what works. The guys (and gals) over at Head-Fi are sure to have opinions and alternate suggestions, maybe even overwhelmingly so.