Just combining a little info here, am I correct that you've had the 'table set up (without touching any adjustments) for over 18 months? If so, I'd go back and recheck *everything*, as in that amount of time a LOT of things can change. I'd start by re-checking the level of the 'table. Floors move, footers can become more/less compliant and change height, racks can settle, etc etc. Then I'd check VTF. Vibrations can alter the settings, even atmospheric changes can affect it. Check the azimuth. Check the VTA -- while the setting may not have moved mechanically, the cartridge's suspension has likely changed at least a little. Clean the electrical connections (ie: rca jacks/plugs, DIN) -- sometimes a dirty contact will manifest itself most in that sibilance frequency range. I won't mention antiskate -- if you haven't been using it all along and had no sibilance issues, continuing to *not* use it didn't suddenly cause the sibilance to appear. :) Finally, although I'm sure you clean your stylus on a regular basis, are you sure it's really clean, and there's not some impacted or baked on crud that you can't see? If you don't have a jeweler's loupe (40x or higher), I'd highly recommend you get one. Not expensive, and chump change compared to what you've invested in your 'table and cartridge.
I don't get the opportunity for as many playing hours as you do, but I normally go through my whole setup every 6 months or so regardless of the play time. And I have yet to go through that ritual without finding something that needed an adjustment. Nothing ever extreme, but the higher the performance level of your setup (and yours is on up there), the more significant even minor setup disparities become audibly.
I don't get the opportunity for as many playing hours as you do, but I normally go through my whole setup every 6 months or so regardless of the play time. And I have yet to go through that ritual without finding something that needed an adjustment. Nothing ever extreme, but the higher the performance level of your setup (and yours is on up there), the more significant even minor setup disparities become audibly.