Here are my sonic first impressions of the Stefan AudioArt Equinox, with some comments on how it compares (to my ears) to its Cardas contemporary:
The Equinox has a very full-bodied sound, with subjectively deeper bass, and greater control in the lower octaves than the stock cable.
Treble extension doesn't seem improved, but refinement of it is evident (and, to my ears, the HD-600 was refined in the treble in stock mode). That is, triangles ring clearer, cymbals hold on to their shimmer better with the Equinoxes (versus the stock cable). Though a fine treble performance with the Equinox, I was hoping for maybe a little more extension, especially since I often listen to my HeadRoom Max in crossfeed-on/filter-off mode, which smooths the high treble a bit.
The crucial midrange region improvement is immediately apparent, with the Equinox-equipped HD-600's rendering all human voices I've pushed through them with a greater sense of immediacy and honesty than with the stock cable. Patricia Barber's voice, singing "Invitation" (on her Nightclub album), has slightly more body than with the stock cable, giving me that "any closer and I'll feel her breathing in my ear" feeling.
Soundstaging is a definite Equinox strength. Stefan AudioArt brags about this on their web site, and it's not fluff -- it is noticeably better and more room-filling (or should I say "head-filling") than the stock cable.
Those are my first impressions, and they're subject to change with more listening.
How does it compare to the Cardas prototype I have? So far I prefer the Cardas cable. The Cardas is definitely more resolving, and, as I stated in my very brief first impressions of it in another thread, is very even-handed in its tonal improvements over the stock cable. Add to that the treble extension granted by the Cardas, and the Cardas HD-580/600 cable is the leader (to my ears) at this point.
With some pop/rock, there can be a slight illusory aural effect of more bass with the Equinox over the Cardas, likely due to what I perceive as a bit less control down there with the Equinox (versus the Cardas). While pleasing with, say, Radiohead's "Airbag" on OK Computer, it can sound just a tiny touch too fat on Massive Attack's opening track ("Angel") on their Mezzanine album. Versus the stock Sennheiser cable, the reach and control in the lower octaves is significantly improved with the Equinox, but it not quite the equivalent of its Cardas competitor in this regard.
As one example of how the Cardas resolves better to my ears than the Equinox, listening to Adolf von Henselt's "Piano Concerto in F Minor op 16" played by Marc-André Hamelin and the BBC Scottish Symphony Orchestra, Hamelin never gets lost under the weight of the orchestra with the Cardas. That is, with the Cardas I have an easier time hearing his each and every note -- even in those short passages in which he's strongly orchestra-accompanied -- than I do with the Equinox. With the Cardas, he more clearly remains the centerpiece of that track when he's playing.
As far as I can tell at this early point in listening to them, the Equinox sets a very high standard, and is a fabulous cable for the HD-580/600. But the Cardas HD-580/600 cable has the edge. So far the Cardas does just about everything at least as good as the Equinox, but edges it out in a few key areas too (namely treble extension, overall resolution, and bass control).
Again, I've not been listening to these cables for too long, so these are early impressions, and subject to change.