Personally, I just hate those STK power modules 'cause they contain all of the tone and preamp circuitry, but my CA-880 was sold to me from a person who died of AIDS, so its the sentimental value of why I like mine and am keeping it. Still works great, but I have not encountered distortion with those paks, so maybe someone else with that type of experience may be of some help.
A power pak could still be defective even brand new, but I would look at those differential pair of transistors and check for out-of-spec resistors in that channel.