So, frequency response is the range of frequencies the preamp can pick up, 20Hz to 20kHz. This part I am sure of.
The slew rate is how quickly the preamp will read a signal. So for instance, a snare drum through an API would not have any distortion (or very little) but say through an Avalon (a "slower" pre) there would be some sort of distortion due to a slower slew rate that cannot keep up with the sharp transient of a snare drum. This distortion is what defines the natural colors of the pre's on certain sources.
I'm more asking than telling :x
