Sounds like you have a lovely space and you have treated it nicely. Do not over dampen the room.
Do you like the sound you are hearing? Do you believe there is an issue or are you thinking that if you remove your racks you just might create one?
Diffusion is already working on the rear wall due to the CD rack so I believe that is the direction to start with.
I would suggest a 6” thick 2x4 panel directly behind you flanked with two panels specifically designed for diffusion. Diffusion is best when they are placed approximately 8 feet from the listener which it appears you will have slightly more so they will work great there.
If I understand correctly the back corners have bass traps all ready so you should be good after the three panels are installed.
Enjoy and don’t over think this since it appears you already have a very nicely treated room.
Live End Dead End treatment works great in a recording studio when you are near field. I have found in my space if the front wall is too dead so is the music. I have bass traps in the from corners, absorption in the middle and diffuser panels directly behind the speakers. Works very well. My speakers are 55 inches out from the front wall measured from the front of the speaker and the speakers are rear ported so the diffuser panel scatters the sound from the port. Just one way to handle a front wall, there are many others, but the key is don’t over deaden the room.