I don't know the Cardas nor the Equinox. The ones I've tried and own are Zu Mobius, Silver Dragon (older and newer version -- slight sonic difference!), Headphile BlackSilver (meanwhile modified), Oehlbach and HD 650 stock cable. All my cables have lengths between 1 and 1.25 m, so Oehlbach and stock cable are shortened.
I've been a long-time Zu Mobius advocate and still think it's one of the best cables for the HD 650, but after changing my source (from Bel Canto DAC2 to McCormack UDP-1) I find it a bit too dark and sharp at the same time, so the Silver Dragon (older version) has become my favorite. Of all my cables these two are the best when it comes to detail and definition. The Silver Dragon is the brightest and leanest, but it's smoother at the upper end and in my now configuration (ICs!) more liquid than the Zu, which in turn has the stronger bass and even higher focus at high frequencies, which is where the SD loses a bit of it compared to the ultra-detailed midrange. If it was possible to attach the HD 650 directly to the amp, without a cable in between, I guess it would sound somewhere between these two.
The Headphile BlackSilver is more forgiving than both and even smoother. It lacks the Zu's and SD's ultimate definition, but offers higher brilliance and a very detailed treble, without being fatiguing. I don't think it lacks bass, BTW, not compared to the stock cable. The Oehlbach sounds smoother, more liquid and spacious than the stock cable, although focus and instrument placement may even be slightly worse. On the other hand it has the stronger bass and higher contrasts, so sounds more dynamic, more brilliant and more fun.
Compared to the aftermarket cables the stock cable sounds a bit lackluster and lacks treble sparkle, clarity and dynamic contrasts, which makes it kind of sallow sounding. I don't care if a cable moves the HD 650 more to the Grado direction and makes it lose its personal characteristic... actually I don't like characteristics at all, and everything that manages to minimize it is welcome, in the interest of neutrality. But since in the real audio world neutrality is actually the product of optimal synergies, there's no sense in insisting on the most neutral cable (hard to define anyway), rather in going for the most synergetic cable instead. Anyway, from their superior definition and precision I would seriously consider either the Zu Mobius or the Silver Dragon, if the system harmonizes with their characteristics. Maybe the Equinox is of a similar caliber, but I haven't heard it. The descriptions of the Cardas make me doubt it is, but who knows. The BlackSilver is an attractive cable for the price and is the most flexible of the top cables.
[Edit: some rectifications, especially in favor of the BlackSilver]
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