You are correct.
The SK25 is best suited to high impedance source like high ratio input transformer. It is a little darker than the SK99 and it is nicer when overdriven. So it is close to the API2520.
The SK99 is best suited to lower impedance source like low or medium ratio input transformer. It is also faster than the SK25, so more open and more detailed. So closer to the 990.
Actually both share the same design and the difference is in the value of 3 resistors. The kits contain both sets so you can experiment.
