may I ask... how sure are you that the former owner did not abuse that k55 by over driving it?
That's a beautiful looking speaker by the way!
post script:
I wanted to also add, that I had a similar situation with a few songs on an old pair of LSii's... I was sure that there was a cap issue or a diaphragm issue... but it only did this distortion/screeching on the same songs, and all other songs were perfect.
it turned out that it was actually the recording and not the mechanics or electronics of the speaker.