where is the soundstage formed

1.where is the soundstage and imaging formed? between the speakers or in your head.

2.does it matter if a stand or rack is between the speakers?

3.I want to put large cabinet between my speakers, how will this effect the sound? the speakers will be about 6 inches forward of the front of the cabinet.
Soundstage is formed via a process called triangulation.

During stereo playback, your brain uses subtle differences in timing and sound level detected by your ears (and present in the stereo recording) to triangulate the positions of the recorded objects.
How wide and tall is the cabinet? A narrow cabinet may not have much of an effect, but a wide cabinet will. A short cabinet (well below the tweeters and midrange) may also be OK.