One other thought is that it’s not the difference on the sidewall distance in your setup, but possibly discovering the best distance between the speakers. If you shifted each in the same direction 1/4” away from the short measurement they would be equal to the side walls and maintain the current distance between. Do they continue to disappear? I have found small changes to the inter speaker distance can be critical in my system.
So, of course, after a hard day’s work, I had nothing better to do than futz around with speaker placement. After some trial and error, I found that my speakers (JA Pulsars non-graphene) work best when pointed straight ahead with no toe-in. When placing them, I inadvertently put the left speaker about 1/2" closer to the side wall than the right speaker. They were previously exactly the same distance from each side wall. With this new, slightly unequal placement, the speakers completely disappear. This was confirmed by my wife who, without any prompting, told me that the music now seems to be coming from out of nowhere, whereas it was directional previously. I can’t explain this phenomenon ... does 1/2" really make that much of a difference? HEY, turn off your dirty minds.Any opinions would be welcome. Thanks.