The Cardas is widely regarded as the mate for the 600. I wouldn’t know, I have been using it fort some time with a 650.
I should begin with saying that I prefer a somewhat darker sound, as a compromise in making classical music sound "real" to me. The stock 650 does a much better job in this area than the only other phone I have extended experience from, the 595, which I find just a bit too bright. It (595) is also clearly more up front in the presentation. Perhaps not as some other phones I have read about, but at least relative the 650.
To the point
What the
Cardas does, is to move you forward towards the stage and the orchestra. The midrange also gets more definition but not necessarily more naturalness IMO. The same with the treble. Mind you, these are not dramatic differences by no means, but still, quite noticeable.
With the bass it is different. Let me first say that I liked the bass of the stock 650 set very much, some think it is too thunderous, deep and prominent, I don’t. With the Cardas, I actually think it is quite the opposite: the lowest bass extension gets restricted till the point low level spatial details get lost. Some don’t mind that - I can imagine: They could not care less if they don’t hear that infra rumble of the passing lorry outside the concert hall. I do. If everyone during the recording even could hear that lorry, I want too
To me the bass of the stock is not eccentuated in classical music. When I first seriously listened to the Cardas after the burn in time (about 200hrs), I first thought the reverb in large concert halls was altered, as if they played somewhere else, I now think more and more it is due to the limited lowest bass extension and a shortened decay-time of notes in general.
In short. I don’t think that the Cardas has a very dramatic influence on the 650 as perhaps some other aftermarket cables have. But that I cannot say out of first hand experience. I do find, however, that it does limit the bass extension to some extend, and that influences the spatial experience: Perhaps some would describe it as the Cardas tighten the bass or focus it better than the stock, but that is not how I experience it. However, if you think the 650 is just a tad too dark and distant, the Cardas just might be a good upgrade for you.