Possible cause of harsh mids?

@noromance 
Yes, sibilance on records is atypical. 
I bought some classical records from a private seller online, all had distortion. It sounded like he played them on an old Victrola.

Does this Corea album exhibit the effect every time when played. How do other albums sound, any harshness?

There’s no mistaking sibilance, It’s a narrow frequency range in the upper mids or highs, the sssss sound. It's sounds different than distortion.
Is the loading on your phonostage set correctly for your cart?

MC has listed the most likely causes if this is a persistent problem.