I only have sibilance issues on a small handful of records. Sounds like groove damage.
Possible cause of harsh mids?
Once in a while you get that used album that sounds good except for a static like distortion on some more dynamic vocals, horns, guitar. Just typical for some old albums? Listening to Corea - The Leprechaun. It's a very good sounding album, well balanced with great dynamics. But, that annoying distortion rears its ugly head once in a while.