Following up from my previous posts I found the following interesting info...
definition: What’s The Difference Between Linear And Non-Linear Loads?
AC electrical loads are referred to either as linear or non-linear depending on how they draw current from the mains power supply waveform.
With a linear load, the relationship between the voltage and current waveforms are sinusoidal and the current at any time is proportional to the voltage (Ohm’s law). Examples of linear loads would include transformers, motors and capacitors.
On the other hand, with a non-linear load the current isn’t proportional to the voltage and it fluctuates based on the alternating load impedance.
Common examples of non-linear loads include rectifiers, variable-speed drives and electronic devices such as computers, printers, TVs, servers and telecoms systems that use switched-mode power supply (SMPS) power conversion technologies. They are also typically found with blade servers.
Having identified what we mean please see the following respected engineering article (no audio vendors here 
)
How bad harmonics influence the work of motors and generators, transformers, capacitors etc
The distortion in the waveform in the
load current of any nonlinear device causes similar changes in the voltage waveform relative to the harmonic impedance of the source network. This voltage distortion affects both the current and voltage for all other loads connected to that system.
Effects of Harmonics
The distortion in the waveform in the
load current of any nonlinear device causes similar changes in the voltage waveform relative to the harmonic impedance of the source network. This voltage distortion affects both the current and voltage for all other loads connected to that system.
How bad harmonics influence on motors and generators, transformers, capacitors and other equipment (on photo: FLUKE 437-II testing equipment power quality tool)
The common effects of such harmonic distortion are as follows:
- Motors and generators
- Transformers
- Capacitors
- Power cables
- Electronic equipment
- Switchgear and relaying
- Fuses
- Communication Systems Interference
4. Power cables
Power cables are inherently capacitive and, as noted above for capacitor banks, their capacitance can produce a risk of resonance with the inductive parts of the network.
These resonance risks and the harmonics themselves can produce the following problems for cable systems:
- Cables involved in system resonance may be subjected to voltage stress and corona.
- Increased heating due to higher rms current, skin effect, and proximity effect. The skin effect will vary with the frequency and conductor size.
Power cable conductors commonly lie very close to one another, and therefore the high-frequency currents in the outer skin of one conductor influence the spread and behavior of high-frequency currents in the skin of the adjoining conductors, giving rise to a “proximity effect.”
The skin effect and proximity effect are proportional to the square of a frequency. Cables therefore have to be derated if there is significant harmonic distortion, particularly if ITHD is greater than 10%.
Of course our engineering colleagues with many years experience will say, as engineers we fix all problems, but in reality we know that nearly all products are a compromise based on cost, tolerances, less than perfect specs and good enough
So
everything is by degree, and given we are looking for as close to 100% perfection not 90%, 95% etc, it is not unreasonable to believe power cables affect audio and visual equipment, true a $100k amp may deal with crap better than a $2 or 3k amp (or cd, 4k player or tv).
PS - re: Use shielded cables for equipment rated above 600 V.
Even my Panasonic tv came with a hardwired 300-500v mains lead and a audio fuse!
