Usually, if you lack imaging you need more absorption in the axis of the imaging problems. Good bass traps work in different frequencies and they aren't located where you need them.
You need to eliminate strong and direct reflections, so start absorbing first. If your image is too short, you need better absorption above and below the speakers and you. If your image isn't deep enough, or clear enough front to back you need more absorption behind the speakers. If your image left to right is unclear, you need more absorption to the sides of the speakers.
Diffusers are usually best behind you or to the sides of your listening location. It's also a matter of taste, but from what you've typed I'd say your issue is not enough quality absorption in the mid to high frequencies in the right location.
Reach out to GIK Acoustics. Great prices with very effective and good looking products, and good help.
Best,
Erik
