I think with regards to the Stanton 500 MKII, the problem is that the suspension is too stiff--could be hardened with age. There is nothing inherently wrong with the Stanton 500 design and the cartridge itself sounds amazing--very open and lifelike compared to a lot of four coil cartridges. The fat cantilevers can fool you. They are actually very thin walled and the main issue there is not size but mass. A cantilever that is fat but of low mass could be more rigid like the lightweight bicycles with the fat aluminum tube frames that weigh nothing and are rigid as hell.
If you have a 500E MKII, though, the VTF range is 2+/-1, which means that while only 1g is "possible," even 3g is still within spec. So, if you have sibilance somewhere under 3g, crank it up to 3 and any sibilance will go away. In the case of a stiff suspension, I think the sibilance is simply caused by the suspension not being able to keep the tip in the groove at the tracking force it is set at. I have Stanton 500 styli of the same model that are all over the place in terms of required VTF; some work great at 1g, others need all of 3g to kill sibilance.
With the right stylus, a Stanton 500 (or Pickering V15) can sound truly incredible in terms of audiophile performance and track lightly, too. Pickering even made a Stereohedron for the V15 1/2" mount and the P Mount version. I have yet to see even a photo of the P Mount Stereohedron for the V15 P Mount, though.