The problem with difference voronoi is that the edges always resort to black, so you don't get transitions from neighbour cell grey values. Ie you don't get a slope from one cell's grey value to the next cell's which smooth voronoi does.
Ideally Ulco is right, it would be great to have an extra input on the smooth voronoi to control the steepness between cell grey values. It's a nice noise to use as is, but can look soft as you've found.
I'd be interested to see some of your test renders of the plain noise to see what you've done.