In case there's anybody else reading this list who is as confused as I was: The Matrix Spatialiser plugin (and others I'm sure, too) has no obvious way to control its value through the gui when used in Ardour or Mixbus.  There is just a text label that says "Width" with a number next to it and no obvious way to manipulate the number.  At first I took the roundabout way and used automation to alter the value.  Then I had my genius moment of the day and realized that right or left clicking on the number would increment the value (use a control or shift modifier for bigger increments).  For some reason this plugin won't store presets but that's not a huge concern for me since there is only one value to change.

I hope this helps make somebody's day easier.

--Steve