The main difference between the "Lidström Improved ZEN i" and the original versions is the current generator on the output.
I have used the same type of N-channel FET as the output transistor. With an N-channel FET the current generator and the output transistor will have the same characteristics according to thermal behavior and distortion.
An additional advantage is that N-channel FET's are easier to find and also less expensive, with a set of equal transistors it is easier to experiment with paralleling of the outputs etc. to increase the power and ability to drive low impedance's.
The difference in sound quality is not very big but an increased resolution in the lower octaves is clear.