I have an action written by Carl Schofield
which allows you to 'sort of ' approximate the blends of 2 curves.
However, I've found that it doesn't help a great deal--my step wedges work as
well. Soft proofing for tonality/contrast, though in grey scale totally,
does work the best for me.
I don't think that Stephen longs for more complexity in QTRgui but
that's the only place where you could make a link between slider
settings and the display. The users should associate the heaviest toning
settings with color samples provided. It could work with a rough color
engine. Mixing with the sliders then shows in the preview too.
Ernst