Slew rate is really "how fast the signal passes through the amplifier". In an amplifier with absolutely no feedback, slew rate is unimportant.... because you certainly can't hear whether what you're listening to takes an extra few millionths of a second to play. (This is ignoring the fact that a really low slew rate probably implies a poor frequency response as well.) However, negative feedback relies on the output signal, which is being fed back to the input, being EXACTLY 180 degrees out of phase with the input signal - and anything less than an infinite slew rate "throws this off" to a degree. Therefore, as you use higher levels of feedback, it becomes more critical that you have a sufficiently high slew rate. To take this to two extremes: 1) If you used zero feedback, and assuming everything else was "theoretically perfect", slew rate wouldn't matter at all. 2) If your slew rate was theoretically perfect (infinite), and everything else was also perfect, there is no reason for ANY amount of feedback to cause audible distortion. (Of course, since nothing is perfect, neither of these two situations ever happens 100%.) There are two reasons why most modern audio equipment doesn't quote slew rate any more. First, this relationship is pretty well known - and it's not that difficult to design an amplifier with a slew rate that is more than sufficient for audio. Therefore it's not really a problem with modern designs. Second, while the particular slew rate that you need to avoid causing problems depends on the design itself, an insufficient slew rate will show up in the normal specifications. (An amplifier with an insufficient slew rate will exhibit THD that rises rapidly at high frequencies. So, if you see an amplifier that has 0.05% THD from 20 Hz to 2 kHz, but has 10% THD at 20 kHz, then there's a very good chance it has insufficient slew rate for the amount of feedback and gain it uses. SInce slew rate varies with both frequency and level, this rise will occur sooner and more prominently as the output level is raised. Conversely, if a design maintains a reasonably low THD over the entire audio band, and at reasonable power levels, then it has sufficient slew rate to go with its other design parameters.) "Back in the old days" the overall distortion specs where high enough that this anomaly would often be "buried" under the normal noise and distortion floor. However, with modern designs, having the THD vs Frequency curve go from "very low" to "almost straight up" at some high but audible frequency would be pretty obvious in the standard graphs... so no separate test is needed.