Typically, the goal of a sound system is to be "a straight wire with gain", in other words, make the sound louder but add or subtract absolutely nothing. Realistically, no system does this, but the closer to this ideal the better, and to get as close to that ideal as possible, every part of the system must be considered. Now, a cable may not effect the sound like a speaker or a DAC, it still can effect the sound; not only because of the cable's electrical properties, but also because of interaction between the cable and the equipment the cable is connected to. And the especially interesting thing about that is that two 'competently designed' cables can have different properties that make the sound coming out of the speakers different.
Consider a speaker cable: The impedance of the cable itself will interact with the impedance of the speaker and the amplifier, what then happens is the speaker's impedance curve will affect its frequency response curve. How strong the effect is depends on the speaker's impedance curve, and the impedance curve of the cable in question and the output impedance of the amplifier in question.
Therefore, there are advantages to using a cable with low impedance. Below is the impedance curve of a Marc Audio Signature Line speaker cable and a typical speaker cable using 16AWG wire. Two 'competently designed' speaker cables that have slightly different electrical properties that result in a difference in sound between the two.
The "typical speaker cable" is the black curve, the Signature Line speaker cable is the red curve. Notice that the Signature Line is under 0.2 Ohms for the whole audio band, whereas the other cable is above 0.2 Ohms from about 7.5kHz and up.
Above is a loudspeaker's impedance curve; the black curve is with the 16AWG cable, the red curve is with the Signature Line, and the green curve is with only the measurement test leads. Notice that the green curve cannot even be seen under the red curve except for a small glitch at 60Hz; so there is essentially no difference between the Signature Line and the test leads. The curves for the cables themselves can be seen at the very bottom of the graph.
Below is the acoustical performance of the loudspeaker used in the above graphs; the measurements were made with the microphone at one inch from the speaker's coaxial drivers, ten measurements were made for both cables (which were removed and reinserted five times during the test) and the results for each cable was averaged to ensure that the results were not an anomaly. The 16AWG speaker cable is the black curve, the Signature Line speaker cable is the red curve. The important part is that the two curves are not on top of each other; in other words, the cable is changing how the speaker sounds. And the divergence between the two curves is sizable in places, being a 0.6dB difference in more than one place. Now, 0.6dB may not be an enormous difference, it is definitely above the threshold of audibility for acoustic differences with the bandwidth seen here.
What People Are Saying
"But, the big surprise is that these mid-level priced cables outperformed my $4,000/pair interconnects." - John E. Johnson, Jr. of Secrets of home Theater and High Fidelity commenting on the Signature Line Interconnects.
Commenting on our Premium Line, Dr. Johnson remarked: "The bottom line here is that for 158 bucks, you can get a pair of RCA interconnects that perform spectacularly!"
The full review of the Premium Line can be seen here.