But I now have, I believe, refined this method slightly, by using Soft Light instead of Linear Light as the blend mode -- that gives much more subtle control.
Well, when you say “more subtle control,” what you mean is that the target layer has less impact on the source layer, so you can move the curves more significantly before going far outside the realm of real colors. Making the control “more subtle” is indeed a good idea, hence the “zoom” layers in my action, posted above.
Soft Light, however, is not the right tool for this, in my opinion. Using Soft Light makes it impossible to lighten blacks or darken whites, and makes the effect of moving the curve by some amount unpredictable.
To see why requires understanding how the various blend modes operate. Here’s a good page for that, with explanatory pictures: http://dunnbypaul.net/blends/