I don't know of any one place that describes each of the Koetsu line. I have read reviews of single Koetsu cartridges in magazines over many years. Manufacturer's/distributors' literature typically praises every product in a way that obscures differences. A cursory Google search turned up the following review of three Koetsu's:
http://www.10audio.com/koetsu.htm
Of course, listening to them yourself would be best, but hard to do.
http://www.10audio.com/koetsu.htm
Of course, listening to them yourself would be best, but hard to do.