Joseph - I've been through this exact same scenario, same table, same problem.
Your problem is not setup related. No matter how much fussing you do with VTA, VTF, alignment, anti-skate, etc. you are not going to fix the mistracking and the sibilance.
Your problem, without any doubt, is you are hitting the limits of the tonearm and cartridge combination's tracking ability. Any form of the JMW-9 arm is going to mistrack with the vast majority of moving coil cartridges.
You can try adding weight to the headshell as Rob has mentioned, but it won't fix things. It might help a bit, but I'm guessing that you actually want to really address the problem.
The best tracking moving coil cartridge that I've found is the Audio Technica AT33PTG. When I tried this cartridge while I still had my Scoutmaster/JMW9sig it about cut the sibilance distortion that I was hearing in half.
If you want to keep your tonearm, you're definitely going to need a different cartridge, and it's probably going to need to be a moving magnet cartridge. Alternatively, take a look at the Soundsmith cartridges. They are supposed to be really good at tracking, and there is one that is special made for VPI tonearms. I haven't tried it myself, but I think you've got a really good shot at clean tracking with this cartridge:
Soundsmith/VPI Zephyr
http://www.musicdirect.com/product/89170
If you want to keep your Benz cartridge (which is a nice cart btw) and have the ability to use other moving coils without tracking error, then you need a new tonearm. The VPI tables are not particular friendly to other tonearms. You would likely need to have it drilled to use another arm, but I do believe that the older style Rega arms are pretty much plug and play. If I were getting a Rega arm however, I would want their newer 3-point mount, which would require drilling.
Switching the table/arm setup entirely is another option. I switched to the Michell Orbe with a Tecnoarm, running with the Audio Technica AT33PTG, and tracking the sibilants nice and cleanly now, even the inner grooves.
Good luck, and keep us posted.