With respect to Acoustat6, it seems unlikely that interconnects or power cables would cause R-channel only sibilance distortion during vinyl playback only. It's easy enough to test (swap cables between sides), but that's low probability IME/IMO.
Elkman,
First, what you're complaining about is "sibilance distortion", not just "sibilance". "Sibilance" means all "s" sounds, distorted or not. Sorry for sounding academic, but technical discussions via written descriptions require accurate terminology.
If this happens with vinyl only, let's identify and diagnose vinyl-related causes first. Aerosans identified one: flawed or damaged vinyl. The other most likely vinyl-related cause is mistracking by your stylus. The first thing to do is determine which of these you're dealing with (if either).
1. Pick a record and passage where the problem is known to occur. Play the passage and confirm the problem is happening.
2. Swap the L and R leads going from your tonearm into your phono cable (or phono cable into phono stage, the closer to the cartridge the better). Play the passage again.
If the sibilance distortion (SD) stays in the R speaker then the problem's not vinyl related. Look into Bshapperd's and Acoustat6's suggestions, or bad tubes in the R channel of your amplification chain, or loose connections in the R channel, etc.
If (as is more likely IMO) the SD moves to the L speaker, you've demonstrated that it's happening somewhere before where you swapped the inputs, ie, in the vinyl rig. Assuming that's the case...
3. Put the connectors back to normal and do the "Slow Play Test". This often proves beyond any doubt that the vinyl is damaged, in which case there's no cure except to replace the LP. Here's how this works: cue the stylus down just before the SD passage with the TT motor OFF but the system powered up at normal volume, or even a bit louder. Spin the platter by hand, slowly, always clockwise. At just a few rpm's the music will be a very LF growl.
If there's groove damage or a pressing flaw, when the stylus hits it you'll hear a MUCH sharper, higher frequency sound, because most groove damage and pressing flaws are more sudden and sharper than the musical signals carved by a cutting stylus.
If you don't hear this after several tries with several SD-prone LP's, chances are your stylus is simply mistracking. In that case, try three things.
1. Clean your stylus. Use Mr. Clean Magic Eraser (after reading the many intructions and cautions in threads on this forum and others).
2. Try increasing anti-skating a bit. R-channel only mistracking is often a sign of inadequate antiskating.
3. If increasing antiskating doesn't help, or if it moves the SD to both channels, your downforce is probably too low. Increase VTF in small increments until it goes away.
BTW, don't make the error that if a little more antiskating is good, more must be better. That is untrue. Antiskating must be set at a happy medium to keep stylus/groovewall pressure roughly equal. Too much is a bad as too little.
Good luck!