Don Hills
Addicted to Fun and Learning
As you said, for all practical cable lengths and audio frequencies, the cable can be largely ignored provided it's a sensible geometry and heavy enough gauge. The fun comes at frequencies in the RF range, if the amplifier is unstable at those frequencies. The gain of an amplifier falls off as the frequency increases. This is compensated for by (negative) feedback within the amplifier, where part of the output signal is inverted and mixed with the input signal. If the output differs from the input due to distortion or gain changing, this applies a correction. The problem is that a signal takes time to pass through the amplifier. If the signal happens to be delayed by half of a cycle, the output signal is now "inverted" compared to the input. Feeding back an inverted version of this signal results in "positive" feedback, where the output is now in phase with the input. The output increases, the feedback increases, the mix at the input increases... This situation is called an oscillator. Designers have to ensure that the gain of the amplifier falls off below 1 before this frequency is reached.
Also, connecting resistors, capacitors and inductors across the amplifier output can cause this instability to occur at lower frequencies. Other effects can occur too. For example, putting a high value of capacitance across the amplifier output will "short circuit" it at high frequencies and on transients containing high frequencies. As well as triggering the amplifier overload protection, this shorts out the feedback signal. The amplifier tries to run at full gain. The usual result is, again, high frequency oscillation. As well as possible IM effects down in the audible range, output transistors are not designed for RF use and can rapidly overheat.
(Yes, you EEs, the above is deliberately oversimplified.)
Also, connecting resistors, capacitors and inductors across the amplifier output can cause this instability to occur at lower frequencies. Other effects can occur too. For example, putting a high value of capacitance across the amplifier output will "short circuit" it at high frequencies and on transients containing high frequencies. As well as triggering the amplifier overload protection, this shorts out the feedback signal. The amplifier tries to run at full gain. The usual result is, again, high frequency oscillation. As well as possible IM effects down in the audible range, output transistors are not designed for RF use and can rapidly overheat.
(Yes, you EEs, the above is deliberately oversimplified.)