Now that the issue is sibilance, I'm wondering if it is associated mainly with the phono and if so also with certain records?
For reference, I detect a sibilance issue most easily by listening to recordings where vocalists, particularly female, pronounce the letter 's'. If the sound is raspy or edgy sounding and not perfectly smooth, then there is sibilance.
Sibilance with the phono is often a result of cartridge misalignment and/or dirt deposits on the stylus (which can be removed easily with a stylus brush) and/or stylus wear or damage.
If it happens with some records and not others, then it is mostly likely record wear or perhaps dirt on those specific records.
If its in the digital source as well, then perhaps there is a loose or poor connection between components or level mismatch issue between components (check component input/out put specs in manuals, if available) or a technical problem within one or more components.
The best way to find a subtle issue like this within a component if it exists is to switch that component in/out of the system with a spare replacement component in good working order, if possible, and see if the issue goes away. Trial and error is involved here.
