Although people may have found in pratice that cables with passive networks on colour the sound this is not the idea. The idea is that the network acts as a balancing element to counter the cables properties. ie, capacitance is balanced with inductance etc. Also there is an element of HF filtering hopefully way outside of the audio band so there is no effect on the phase response at what could be considered audiable.
The problem I have with this is that purely by adding any components to a cable you will colour the sound in some way, although you may be able to measure the "corrective" effects.
The other thing with passive networks is that the result in the real world is inconsistant as it depends on the source and receiving impedances.