When I use Ardour 4.0 Midi tracks with Linuxsampler it seems the R/L-Stereo setting of the tracks is locked (like it is when you use stereo effects). How can I nonetheless influence the R/L position of the sound? I am using qsampler for loading instruments but seem unable to find an R/L setting there either.

You have to reduce the width of the stereo panner to less than 100% before you can adjust it left or right.

Read more here: http://manual.ardour.org/mixing/panning/stereo_panner/

Stereo tracks in ardour have a pan control which lets you set the stereo width, as well as the position in the stereo image. Drag the “L” and “R” ‘handles’ on the pan control to widen or narrow the width of the stereo source, you can then move the pan control left and right to place that stereo source within the overall stereo image. By definition, if the width of the source is 100% of the stereo image, you can’t move it left and right within the image

If you’re running linuxsampler “outboard” (i.e. not as a plugin/instrument), you also have the option of patching it to two mono tracks, and panning them independently.

(Is it even available as a plugin/instrument? I’ve never looked.)