PRR, you forgot to mention inductance of cables. Actually, they represent both distributed capacitance and inductance, and have a transfer impedance that depends both on capacitance per length and on inductance per length of the cable. On frequencies where length of cable is comparable to length of the wave we can't say about which reactance is dominant without measuring the cable length; rather we have to load the cable on both ends with certain resistances, otherwise reflections will occur from points of an impedance mismatch causing comb-filter effect.
Here is a good article that describes the theory of long lines:
http://www.epanorama.net/documents/wiring/cable_impedance.html
However, for short cables of 100 meters of length or less on audio frequencies capacitive reactance will dominate, so we may assume a capacitive load of opamps driving cables.
Opamp has output resistance, that with capacitance of a cable will form a R-C network (so called integrating network), or low-pass-filter. What is worse, output resistaqnce of an opamp is non-linear, it means that the lower is load resistance, the more distorted will be output signal.
Also, such a network consisting of a resistance and capacitance shifts phase up to -90 degrees. Now, assume that every transistor stage shifts phase on high frequencies, sum all phase shifts, and get 180 degrees starting from some frequency. If on that frequency an amplifying factor in the feedback loop (you remember, that opamps in audio are always used with a negative feedback) is greater than 1 AND phase shift is 180 degrees it means that on such a frequency a negative feedback becomes positive and your amp starts to oscillate! I.e. instead of just amplifying an audio signal it also generates a radio frequency signal!
Now, since output resistance of opamp is non-linear, as I mentioned before, an amplitude of oscillations will be changed by an audio signal, so as the result you will hear DISTORTIONS!
To prevent such oscillations a cable may be connected through an inductance that has the higher impedance the higher is frequency, right?
Not so simple... Inductance and capacitance in series form a L-C contour that on certain frequency has almost zero impedance, i.e. output of the opamp will be almost shorted on certain frequency. Below this "tuning frequency" a capacitive part will dominate... Now, you will have a radio transmitter that works on more stable frequency than before, firing distorted audio signal around.
What to do, how to solve the problem?
Add a resistor that will absorb an energy from the contour making it's selectivity less sharper. Either in series, or in parallel. As the result, a phase-angle Q of the L-C contour will be lowered, it is exactly what that guys are doing adding what you call "an isolator". Connecting in series you will increase output resistance of your amp. Connecting in parallel with capacitance of the cable you load your amp on audio frequencies. Connecting in parallel with an inductance you lower impedance on very high frequency that is inaudible. Such a simple theory. :grin:
