I should perhaps add that it is possible to construct audio equipment in such a way that small cable differences can be magnified.
The principle of audio reproduction is that the voltage coming out of the amplifier represents the amplified original voltage signal (from a recording) as faithfully as possible.
Audio equipment therefore should (in theory) have low output impedance compared to the load being driven (high input impedance) - and it is this "rule of thumb" which preserves the accuracy of the voltage signal between components. In this case, variations in the load become insignificant to the output device and there is negligible signal voltage loss across the output impedance of the driving amplifier: the load sees what it should.
Now ignoring cables for a moment, imagine a load with large varations in impedance with frequency, say from 20 ohms to 1 ohm. Now connect this to a 10 Ohm (high) output impedance amp. The voltage drop across the output of the amp at the lowest impedance point of the speaker will be 10/11 or roughly 91%....this means a mere 9% of the intended voltage actualy reaches the speaker. At 2 ohm load the calculation becomes 10/12 and 83 % of the voltage drop is across the amplifier and the speaker sees 17% of the intended voltage signal. In this case a 1 ohm variation in load causes a near doubling in voltage seen by the speaker. At the frequencies where the speaker load impedance is highest (20 ohms), the drop across the amplifier will be only 10/30 or 30% and 70% of the intended drive voltage will reach the speaker... this is nine times more voltage then at the lowest impedance point.
If you do the same calculation for a 0.01 output impedance amp you will conclude that 99% of the drive voltage reaches the speaker in all cases.
The above is mere "back of the envelope stuff" or gross approximations but it establishes that a high output impedance device will change significantly its driving voltage in reponse to small variations in a low impedance load. And that, unfortunately, includes slight inductance, impedance and capacitance from speaker cables too...in this situation it can become a not entirely negligible factor.
I would suggest that there are ways to design audio equipment so that cables have only very minor effects: speakers that do not have nasty low impedance dips and power amplifiers with low output impedance under all frequency conditions; it is these two conditions are mainly required to minimize the effect of cables. This is also a well known rule of thumb.
