Sibilance -- that last little bit...

I hear only two issues:

1. Cartridge went bad
2. Damaged pivot on the arm.

Try another cartridge to make sure that yours is OK or not.

"I have the ever so slightest bit of sibilance toward the end of some LP's."

This sounds to me like plain old end-of-side distortion. If the cartridge is perfectly aligned then it may be inherent in the LPs. Which protractor do you use ?

Have you played any of the offending LPs on a "reference table" at a friend or dealer to see if the problem might be in the LPs. It may be worth doing this before spending much time effort and money on what might be a futile mission.

Since you did not volunteer it, let's guess. Grado?

Here's a link to another post that mentions the same problem.

http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1101531954&read&keyw&zzwell+tempered

I have a WT classic and don't have the issue. I used a WT protractor for cart alignment, and the a hifi news test record to set up the cartridge for azimuth and anti skate, etc. The test record was very good for the anti skate. Another A-gon member (red) posted a good method for setting up the azimuth due to the dished platter. hook up a tape player or cd burner (anything with a left/right channel sound meter) and use the 300hz track to adjust the azimuth until both channels are equal levels. Its surprising how the cartridge appears to lean right, but both channels on the sound meter have the same output.

Let us know how it works out.

Gibson 58, I have to question this method of setting azimuth. If the cartridge output is not perfectly matched, channel to channel, you will be correcting the wrong thing. Maybe that's why you are leaning to the right. Try just doing it by placing a toothpick on the headshell, or placing a small mirror on the platter itself.
Marty