Sean, while I usually find your responses to be well reasoned, I find nothing in this one to support your position. Just what do you mean by " circuit degradation?" You state that it occurs, but offer no explanation of how it occurs or what form it takes.
What characteristic of any component will change? I can see the argument for power amps if the speaker is part of the feedback circuitry (very unusual), or in the case of inductive kickback from unloaded output transformers in tube amps, but for line level devices there is nothing I can see that is affected. Too low of an impedance can cause degredation of components due to heat, what can too high do? I say nothing.
The overwhelming majority of line level pieces use an emitter follower output stage. That puts the input impedance of the following stage in parallel with the emitter resistor. Without the load, it is driving current through a 600 ohm or so emitter resistor. With the load, add the input impedance of > 10k ohms in parallel, which is insignificant. Other circuit topologies can be analyzed in similar fashion.
