Absorb or Diffuse in between speakers?

Diffuse is best.

I have heard cone speakers and horns sound best when there is some absorption between them, meaning the image solidifies in the center and the depth increases. However, with panel speakers diffusion seems to work best.

Unfortunately, I think the real answer is - try an inexpensive method to test both and wee shat works for your situation before you buy something more permanent. Every room is different.

"Unfortunately, I think the real answer is - try an inexpensive method to test both and wee shat works for your situation before you buy something more permanent. Every room is different."

Good answer. Agree with that.

I am all for experimentation first. In my own setup, I went with a panel that uses absorption on the sides and a small hard diffuser in the center. This worked well for me.

A friend has a room that was designed by Rives. It has a very large, convex, curved, hardwood diffuser centered between the speakers.