[Eliot Ness]

I'm reminded of a Zenith console I was working on several years ago that also had a distortion issue.  After much hair pulling I noticed it used the same speaker as a parts console I had.  On a whim I plugged in the other speaker and BINGO, the distortion was gone (and the offending speaker looked perfect).  It's probably a long-shot, but if you can substitute in another speaker it would rule that out.  Ever since that experience I keep a speaker substitution box in the shop:

   
Photo courtesy of Steve Johnson: https://stevenjohnson.com/testers-misc.htm