Transparent Audio's website says that the network boxes control high-frequency roll-off, attenuate radio frequency noise, and alter the capacitance and inductance of the cable to account for changes in these parameters as the length of the cable changes.
http://www.transparentcable.com/design/audio_cable_networks2.php?catID=1&modCAT=1
Do the Transparent Audio network boxes also change the input impedance a receiving component sees at its input?
Do the network boxes also somehow better match the output impedance of a sending component (such as a phono pre-amp) and the input impedance of a receiving component (such as an amplifier)?
Having designed speaker cables with these sorts of devices, but not knowing exactly what is in their boxes:
The speaker impedance is usually around 4-20 ohms. These devices will not change that significantly. It will have no effect on this.
The output impedance of the amplifier driver is not "matched" to the input impedance of the speaker by these passive components. The kind of "matching" your are suggesting is not relevant. The best scenario is 0 ohms output impedance and a reasonably high speaker impedance, like 4-10 ohms.
I can tell you what my "Black Boxes" do, but I will not divulge what is in the box. Some, and usually the best speaker cables have high-Q characteristics (high-Q means it will ring like a bell when stimulated with a HF transient). These types of cables are usually very low in inductance and resistance.
Transients in the music will cause reflections and standing waves on such a cable. The transients in the music are relatively low frequency compared to the HF effects in the cable, but they still occur. This is kind of like hitting a bell with a hammer made of rubber or soft wood. It still rings, but not as loud as if you hit it with a piece of metal. The hammer is the music and the bell is the cable.
To squelch this ringing, some manufacturers put filters and HF attenuator circuits in their cables. This works, but also impacts audio quality by rolling-off highs or affecting transients. The real solution to this is a "tuned" circuit that nulls out the reflection without impacting the audio frequency range. This is what is used in a transmission-line that is properly terminated. The termination kills the reflections usually by absorbing them. As far as I know, most manufacturers don't understand the transmission-line effects, so their solutions are useful, but not optimal. There is generally a sacrifice in SQ for reducing this effect.
Why do we care about reflected energy on the speaker cable? Because it drives transient current back into the driver, it can cause the driver to behave non-linearly. This can be audible.
Steve N.
Empirical Audio