Rise Time is very much the same as "Slew Rate" – the rate of speed at which the amplification circuit can amplify the signal. You can’t go from 0 to 5 volts in no time – and how fast a circuit can amplify is part of what imparts its sound. Vintage circuits were much slower than are today’s, and in theory, faster is better. Faster amplification circuits retain the leading edge of the transient signal, especially apparent on the higher frequencies. But slowing the rise time down can mellow or smooth out the signal, often rounding out harsh sibilants from vocals or edgy tones of instruments. In a way, you can think of Rise Time as a "Time Machine" for preamplifiers – the slower the rate, the more vintage the sound. No other preamp has this special feature.