What is the Sound of Impedence Mismatch?

The impedance mismatch is only critical when the mismatch causes the preamp to go into distortion. If the preamp's current drain into a lower impedence device puts the preamp into a clipping mode similar to a amp driving low impedence speakers at high volume. So the sound is normal distortion IM and THD. Harsh sound would be the first clue. Some preamps can take a ratio of 1 and others need an amp impedence of 20 time their output impedence because they can't "drive" any current.

Yes Bob I am familiar with LCR effects in cables. What's your point? I was answering the question about preamp amp interaction without considering cables. Poulkirk brings up the professional 600ohm standard which while correct is likely not answering the question which is probably possed from a RCA connection perspective. I was addressing the loading issue, not a potential frequency rolloff due to inductance effects. You might want to explain the issue of frequency rolloff to Drubin and how the R in the LCR circuit is partially the output impedence of the preamp since you are so keen on it.