I tested Volcano today. I thought, it would be possible to assign drive to selected frequencies only - but it seems, harmonics are created over the hole spectrum. Any impressions on that?
The drive knob simply increases the input to the filter and decreases the output of the filter (or vice versa when using negative drive) in order to increase or decrease the characteristics of the filter style. So when using the drive with a bell filter, you will indeed increase the level of the complete audio spectrum going to the filter.
The shelf filters seem to be focusing the harmonics more to the area covered by the shelf filter itself, so you could have the first shelf filter with a coloured style as "pre-emphasis" and drive, and then use another filter as "post-emphasis" with a clean algorithm. That's how I got best results with it as a saturator.
I guess if you want to distort just a super specific frequency, the best bet is to use a bandpass filter in that area and drive it heavily – then use the global mix control to blend it with the full-spectrum unsaturated signal. You could even use clean filters in series after the driving filter to remove unwanted frequencies from the saturated signal. So kind of like parallel frequency specific distortion. Or you could just load up Saturn 2 and do it there, as it seems more geared to do this sort of thing. :)
Just wanted to add a disclaimer still, that I'm basing all these on my experience with the plug-in after testing it for a while. For the FabFilter guys: if there's something factually incorrect or unsound, feel free to correct me here!