Choosing cables for Passive applications?

I think a better cable (in your system) is a better cable rather than a cable having lower capacitance etc. being better. Maybe I just don't understand or care much about the technical aspects of cables and what makes one cable better than the other. I can tell you that the Nordost cables have such a different sonic signature as the Cardas that you're comparing something that is quite different.

With passive, the biggest thing you have to worry about is the quality of the passive pre and the length of the cables (especially the length of the cable connecting your amps to the preamp). Maybe capacitance, low resistance, and low impedance all play a role, but I would still say trust your ears.

Yes, the Nordost cables do have much lower capitance. Knowing this, I have used both Nordost Blue Heaven and Red Dawn with superb results on my Reference Line Preeminence 1, Series 2. While I never had Cardas for comparison, I can say that the Nordost TOTALLY blew away some comparably-priced Audioquest. The Nordost are so amazing I can't imagine using anything else now.

Another thing is that plagues passives is improper system matching. This, and or long cable lengths can result in the lost of dynamics and a muddied/softened representation. Besides Nordost, there are many cables that will work well in a passive system, it's just that cables that tend to be sharper in focus tend to help compensate for the deficiencies. When done right, passive certainly sounds very good.

My system uses a passive preamp and the runs are as short as possible (30 inch & 1m) Ridge Street Audio MSE's.

Short lengths and low cap is desired on passives. On my second system, I have the Placette passive line stage connected to a Fourier OTL. I use the Pure Note Cerulean cables that maintain good bass response and speed. Nordost (Valhalla) IME is way to thin and bright.