Well, what are you using to drive GS? What sequencer (or MIDI)? Look at the midi-mixer screen and note where the volume control is on the individual instruments. If you click on the right side (I think I'm doing this from memory right now), it will pop up a list of midi controls. You can set up each one to something different. Then either using real controls on your midi controller bound to these settings, or by inserting the appropriate things in your sequencer/notation program, you can change these values.
I want to be able to cross fade a p instrument, into the f version of teh same instrument, and I want to do it via the stacked instrument capabilities of GS3 if possible, rather than doing an edit of the instrument
It can't be done. There are two reasons: First GS3 only lets us automate the overall volume for the channel. Second GS3 doesn't let us invert the controls.
I just took a look at the automation for GS3 and it seems to have taken a big step backwards from GS2.5. This is really unfortunate.
If you open NFX1 in the DSP section and right click a control you can automate it. You used to be able to invert the control and set the control range. Now these features are grayed out.
I'm hoping that these features come back soon. Lets say you want to tie a high frequency shelf to the expression control. You can do that today, but the only possibility is to have the shelf move at full range with the expression. That may be too much, but we can't adjust it with the current limitation.
Let's hope that inversion and scaling are implemented in a future update. We had the features in GS2.5. Hopefully they left 3.00 only terporarily, due to schedule pressure.