Regarding the cables discussion. A couple of plausible theories, assuming that neither confirmation bias, nor slick salesman-induced hypnosis are at play.
(A) A cable is also an antenna.
I observed mobile phone interference with a studio monitor once. Manifested as a periodic crackle in one of the monitor's three transducers. Moving the phone from ~1 feet to ~3 feet from the studio monitor resolved the issue.
A well-shielded cable, especially with carefully twisted identical wires, is less of an antenna. Thus, at a location with a strong RF field, it could theoretically provide a protection from the interference, which could otherwise induce distortions.
To test this hypothesis, the cable and equipment would need to be moved and turned around, let's say several feet away and ninety degrees, to potentially change the interference effect.
(B) A cable is also a heat sink.
Imagine a thermally-challenged piece of equipment. Could be a compact tube apparatus. Or perhaps a vintage solid-state amplifier with a dried-out thermal paste between the power stage transistors and regular heat sink.
Massive enough cable, made of materials with high thermal conductivity coefficient, and with a tightly inserting connector (perhaps even slightly lubricated with electrically and thermally conductive paste), may cool off at least the power transformer coil to which the power supply wires are connected.
Cooled off power transformer coil would then "extract" heat from other transformer coils, which are connected to rectifier on the printed circuit board, from transformer core, and so on.
Also, some heat could be extracted by convection from the air circulating inside the case shared by the transformer with other amplifier components, cooling down even components situated far from the transformer.
In effect, such a cable could serve as an auxiliary heat sink, analogous to a transmission cooling radiator on certain high-performance cars and trucks. The analogy extends to potential positive effect from increasing air flow around the auxiliary heat sink. In case of the cable, it could be achieved by lifting it off the ground.
To test this hypothesis, one would need to measure change in equilibrium amplifier temperature at the same settings and audio material with one cable vs another. The equilibrium temperature is the one that no longer rises, after some time since the test was started.
