Hi capacitance cables may be used to 'tune' a too bright system.
That is true in the case of interconnects, especially if the output impedance of the component driving the cable is high. It is not true, though, for speaker cables, at least to the extent that cable effects are explainable by generally recognized science (and assuming that the cable does not include a network box of unknown design).
The reason that interconnect cable capacitance can roll off high frequencies is that it interacts with the output impedance of the component driving the cable to form an RC low pass filter (the "R" term corresponding to the output impedance, and the "C" to the total capacitance of the cable). The 3-db bandwidth of the filter (the frequency at which the signal is attenuated by 3db relative to its amplitude at low frequencies) equals 1/(2 x pi x R x C).
In the case of a power amplifier, output impedance is near zero (a small fraction of an ohm for solid state amps, and at most a few ohms for nearly all tube amps), so that effect will not be significant for audible frequencies.
High inductance in a speaker cable, on the other hand, can reduce brightness to some degree, by introducing an impedance that rises at high frequencies and that is in series with the speaker impedance. The inductance of even an average cable can have that effect, if cable length is long and speaker impedance at high frequencies is low.
Best regards,
-- Al
