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.

I also have a fireplace (fake) with a glass front between my speakers (Silverline Preludes). The speakers are about 8 inches further into the room than the (still fake) fireplace, and 7 or so inches from the edge of the aforementioned (fake) fireplace, so reflections are gonna go sideways and it doesn't seem to hurt soundstaging, imaging, musical content, phase, or lyrical sensibilities (maybe that one a little). I've pulled them into the middle of the room and otherwise tried various degrees of tow-in, height adjustment, spikes, etc., and they sound best pointed straight forward raised up on butcher blocks with vibrapods under them. Also, the little corner the fireplace creates behind the speakers is perfect for a little REL Q150e sub. This information is useful only to me or those exactly like me with identical gear in a room exactly like mine.