The Class A Amp was originally added to the DP/4+ and was then ported for the ESP2 chip for release in the DP/Pro. It emulates the distortion of a Class A guitar amp and cabinet. The Tube Bias control adjusts the odd/even harmonic ratio, and combined with the eqs, the amp is a true chameleon. I honestly can't believe how good this sounds. Also, it's invaluable for being able to plug a guitar into PARIS direct for reamping later, while stil having an amp sound. Version 1.4 includes an input gain control and introduces an improved cabinet emulator.