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.
1. I believe imagaing is the "invisible" focal point where the music produced from a pair of loudspeakers meet in the middle. It could also be a center point where music from a surround sound system meet as well. It's where the speakers disappear and the sound is in the middle of the room so to speak. Of course it's in your ears and head where it ends up !!

2. Soundstage has more to do with how close you feel you are to the music (from the stage so to speak).

3. Just my humble opinion but I would minimize large sound absorbring or sound reflecting solid objects between loudspeakers. Keep the area relatively open so the sound is free to image properly. Keeping the speakers forward of the objects will certainly help somewhat. Just my opinion.

There's much to consider when setting up a listening room for your sound system. I believe the room set up and arrangement is as equally important as the audio system itself. If your room is too alive for example, you need to add bass traps and absorbing materials such as upholstery, carpeting, drapes. If it's too dead, you need to open it up and remove these types of items.
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.