According to an older Sams Tube Substitution Handbook I have, the 25A6 can be substituted for the KT33. According to references on the web the KT33C is identical to the KT33 except that the KT33C provides a filament center-tap connection on pin 1, while pin 1 is unconnected on the KT33.
Therefore I suspect that settings for the 25A6 will work for the KT33C. I don't guarantee that, however :-)
Regards,
-- Al