Sorry my friend, I just realized now how old this thread is! And thank you for providing more details. But I am talking about the line level output and input. On wikipedia and many webistes, is said that output impedance of line level should be at most 600 ohm because most input impedance of input line level is usually 10k ohm. And these people always say is highly advisable to keep the 10:1 ratio of output impedance / input impedance of line level in order to acomplish good sound quality.This is a really old thread nowand I can't remember what it was all about but I think you might be confusing output impedance of line stages (which are usually low) and the loading of a 10k pot across that output.
10k is a good starting value for most line level type stages that need a volume control on the output.
However in this thread, there are many people saying that to control the output line level people should use a 10k potentiometer... that would be very far away from the recommendation of many experts. So I ask you: is there an audible loss in audio quality when using the 10k potentiometer instead of 1k ohm? I think 1k ohm would be much better suggestion, right?
600 ohms is pretty high for a modern line output stage but that statement is essentially correct.
Typically a line output level is from an opamp (output impedance of milliohms when configured) and an additional series resistance of up to say 100 ohms is often added to allow for muting functions to be applied and to maintain stability into adverse loads (high cable capacitance)
If you use a 1k pot with a 600 ohm output impedance you will instantly lose almost half your signal. For every volt of preamp output the 1k will load the output and reduce it to just over 0.6 volts. The stage may struggle to drive such a low impedance and distortion will probably rise as a result.
Look at any preamp or integrated amp and most volume controls are in the 10k to 100k region.