Impedance Mismatch ???

usually impedance is measured at 1000Hz signal that may show different values.

There are certain amps, such as the Krell Cast stuff that have the preamp acting as a current source, rather than a voltage source:

http://www.krellonline.com/cast.html

If this is the case, preamp output impedance and amp input impedance must be exactly the same. But this is a very unusual circumstance.

For most hi-fi rigs I actually look for a minimum ratio of 20:1.

Viridian, thanks for the link - very interesting. Current loops like 4-20mA became at one point industry standard for their inherent immunity to noise or resistance of the cable especially on very long runs. Current source has very high output impedance making more difficult to induce currents in the loop.

I am currently running an amp that has 600 ohm input transformers. This tube amp does not match up well with anything but a linestage that also has 600 ohm output transformers (I have such a linestage). A friend has a different amp that also has input transformers (also 600 ohms) that is hard to match with linestages. None of his tube linestages work well with that amp, and even solid state linestages that do not have an extremely low output impedance is not ideal; he runs that amp with a Levinson No. 32 preamp which has a very low 20 ohm output impedance into unbalanced lines (10 ohms into balanced). This is another example of rare, special requirements.