Vinyl and slurring S'es

The sound that you are hearing is formally known as "sibilance".
A number of good possibilities are mentioned above; may or may not be the issue here, so you'll have to experiment.
The cause may also be dirty/un-conditioned AC power, the wrong interconnects, high-level overloading of the phono input stage lineup, the wrong AC cord.

Paulwp hit it on the head. The more precisely that you set up a cartridge / arm / phono stage, the better the sonics and less wear & tear on the vinyl and stylus.

Also keep in mind that cartridge loading can drastically affect tonal balance and apparent sibilance. The cartridge loading is determined by EVERYTHING between it and the phono stage. This means your internal tonearm wiring, the interconnects and specific values inside the phono stage should all be taken into account when trying to calculate total loading values. Sean
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I agree that it is most likely the VTA. I prefer the arm to be slightly lower at the pivot. This usually increases bass output and smoothes out the highs. The tracking weight can also influence the top end as Paulwp stated. It is not always best to choose the lightest that the manufacturer recommends. Go for the middle range. It will cut down on record wear and mistracking.

Try this; turn the volume all the way up to listen to the background noise of the cd and the analog. If you get a lot of hiss in the analog, it may be an indication that there is something going on with your phono stage or interface between cartridge and phono. It won't give you absolute results, but it may lead to electronics and wiring as oppossed to tracking.
If you do get a considerable amount of hiss, substantailly more than the cd rig, try some different ic's and different connections to see if it changes. Try to eliminate as much as you can in order to isolate the anomoly, no matter how simple it sounds. I would evan unplug my ic's from the back of my cd and plug them into the phono and visa versa, just to see if it is the sound of the ic's I'm hearing or something in the pre-amp that is different. It's hard to tell from here if there is something wrong, there is just a synergistic mismatch of components, or if it's a matter of taste. From there, you might have a better shot at finding out which it is.

I have noticed this with 180g vinyl discs. The slurred and hissing/rough S's are really annoying (e.g., Dylan's "Highway 61 Revisited"). In fact, I hadn't played any for some time. Now, I am wondering if these discs require some modified VTA, stylus force, etc.

Anyone else have this problem and a solution? Thanks!