What would you say is THE MOST important factor to good imaging?

Well that's that then... Let it be known horns do not image (looking for gavel to hit table)

#1 would be getting rid of all reflections in the room . Obviously not all . Third wall reflections and on will not have much energy anyways . Try it out . Buy some Owens Corning or Johnsmanville 2'x4' rigid insulation as a test in your room . Under $100 for a 9 sheet pack where I live. Buy 9,18,27 sheets . Do your testing . Keep the stuff or buy professionally . I have moved a few times recently and listed hundreds of sheets on craigslist and sold it everytime I listed it at 3/4 of its retail . 

Program material/mastering. If it’s not there, no speaker placement, no cables, no room treatments, no electronics will create it or correct it or enhance it.

Some of the best imaging setups I have heard, the speakers were extremely far out into the room, as in dedicated listening space.

I keep my TAD’S on the long wall, almost five feet into the room (measured from the rear) since it’s not a dedicated room and can’t afford more space behind without making the listening distance too close.

The benefit of this setup is having them close to 10’ from the side walls.

When I first purchased the loft space, was planning on placing them on the short wall to allow a little more space behind the speaker, and a more distant listening position.

However, I had THE Andrew Jones look at the raw space before I moved in and he suggest the long wall setup, and I who am I to argue with a trained physicist. 

 horns just have a diffused "wall of sound." 

Utter nonsense. If you can’t get horns to image, you are doing something wrong. And horns are a lot more directional than planars. You are very confused.

Oz