If esses are such a big problem for you, your problem is probably the vocalist's anatomy or your recording chain.
If he/she has a visible gap between his/her front teeth, that might be the cause of harsh S-ses. Try filling the gap with some chewing gum - sounds funny but works great.
Like this:
OTOH I'm quite happy with the Voxbox deesser, although I don't use it very often - better try to get the right sound with the right microphone (position).
What's your recording chain (mic?)