Good points, 13DoW. A solid Teflon dielectric tends to be better implemented for a cable application that is immobile, with little or no handling involved that would disturb the relationship of the conductor and dielectric. I would also avoid using a litz wire cable for a headphone application for the same reason. I also agree with the notion of individual cable runs for each channel rather than a twisted triad stereo cable with a shared return/ground conductor for both channels.
If I were to upgrade a stock headphone cable with a much higher-quality cable, I would consider the VH Audio V-Quad cable featuring a cellular FEP dielectric that might be less prone to dielectric/conductor disturbance than a solid FEP or solid PTFE insulator. It also features a twisted quad of 24 AWG (21 AWG aggregate) solid core conductors with lowered-inductance and a better EMI/RFI noise cancelling geometry vs. a twisted pair. If one wanted to choose a lower-cost option, a single V-Quad cable could be used as a dual-channel headphone cable, with an individual ground conductor implemented for each channel.
