I posted a thread about this a few years back also. I've noticed that the image can shift on me daily and I've attributed this to my ears. Some days my left ear is more clogged up, sometimes it's my right ear. I've made cleaning my ears part of my audio hobby. I also value my hearing more than ever and wear ear plugs at times to protect my hearing (15,000 rpm's and 100mph wind is no good for the ears).
I've found that the bay window in my room accentuates the bass in the same way that bass is louder in the corners of the room.
Since you are so exact with speaker placement you ought to do what I do and attach a string to the center of the headrest on your listening chair and make sure that your speakers are equidistant from the listening position. I've found that to be more critical than the speakers being the exact distance from the walls (someone pointed out to me that rooms aren't perfectly square). You still need to "eye" the placement because they can be equidistant but not equally apart from the centerline. I also use a level to level my speakers.
I've found that the bay window in my room accentuates the bass in the same way that bass is louder in the corners of the room.
Since you are so exact with speaker placement you ought to do what I do and attach a string to the center of the headrest on your listening chair and make sure that your speakers are equidistant from the listening position. I've found that to be more critical than the speakers being the exact distance from the walls (someone pointed out to me that rooms aren't perfectly square). You still need to "eye" the placement because they can be equidistant but not equally apart from the centerline. I also use a level to level my speakers.