[Jack the cat]

NM

[mick wolfe]

The same issue with nearly the same size room. And yes, that room was a sonic disaster until I slowly introduced room treatments. Since you'll be forced to listen near field because of the room size, that's a good start. I started with 2'x2'x2" absorbing panels on the front wall then added diffusion on the back wall. Recently added some diffusion on the front wall near the corners to good effect, still retaining the original absorbing panels.  Bottom line is that you have to "deaden" the room to the point where it reasonably passes a clap echo test. And yes, you can over do it, so proceed slowly.

[eichlerera1]

The same issue with nearly the same size room. And yes, that room was a sonic disaster until I slowly introduced room treatments. Since you'll be forced to listen near field because of the room size, that's a good start. I started with 2'x2'x2" absorbing panels on the front wall then added diffusion on the back wall. Recently added some diffusion on the front wall near the corners to good effect, still retaining the original absorbing panels.  Bottom line is that you have to "deaden" the room to the point where it reasonably passes a clap echo test. And yes, you can over do it, so proceed slowly.

I've found you can deaden a room too much with the overuse of sound absorbing panels, but you can't over diffuse a room with the use of sound diffusing products.....

[JLM]

The challenge with diffusion is that the laws of physics limit the frequency they work by the depth of the panel.  Most commercial offerings are no more than 7.5" deep which equates down to 1100 Hz.  You mentioned having stuffed bookcases.  I use randomly filled bookcases on side walls as "casual" diffusors.  If you pull some of the books out you could in effect have diffusors.  Books make good diffusion material too (dense, non absorptive).

[VinceT]

I also deal with this to some degree. Lots of good suggestions here. I was able to get my speakers to disappear pretty good by moving them about 1/3 into the room. You can also move your listening position further into the room, or do the opposing corners approach with putting speakers and listening position in opposite corners. A few inches here and there with speaker positioning and listening position makes a big difference. What I did is got out the measuring tape and paper and started charting positioning and seating arrangement locations. I think I found what is best after a couple months or so of trail and error. Once it sounds really good, room treatments only will make improvements. Try to let the room dictate positioning not furnature placement...if possible. I took the approach of breaking up symmetry with placement. Speaker postion in the room, position from the back wall to side walls. Position with seating position off the back wall, speaker toe in, speaker height..... 3rds, 5ths, 8ths, etc.
Also want to mention while doing the process I was mindful of speaker volume and cut out some of the bass, since bass modes are most affected by a square room in order to obtain the best imaging. Once I found the right placement I was able to introduce more bass. I also just played bass in these positions to see what ares would overload the room to rule out those placement options.
 

Anyway...I am by no means an expert at all and new to this hobby. Much what I have learned is from the great knowledge of this board and other online resources,  so take what I say with a grain of salt. Like you I have been doing much research on the subject and was able to get my speakers to image and disappear by doing this process in my room. Hope this helps.

[timind]

I mean arranged perfectly symmetrical to the corner, I think like this example - https://images.app.goo.gl/7GdkdoAiArUtX4V9A

Right now they are symmetrical to the back wall, and wondered ignorantly if I’d be trading one problem for another.

I wouldn't call 10x12 square. That said, since you referenced one of my pics here, I'll say using the diagonal set in my small square room (12x12) was the best sound I had in any small room. Not pictured are the bass traps behind the listening seat, and bass trap in corner to right of speakers. The bass trap behind my seat worked wonders to clean up the bass.

The pics are from 5-6 years ago. Here's another: