You can't test things like this with one side only. I've tried many times with damping changes to orthos and all it does is confuse you and ruin your memory of how they sounded before the change. The human brain just cannot compare left to right like that and there's too many variables with recordings. So I don't think you can trust what you hear or don't hear in this case.
I believe you are right because I also tested a new leather pad that I made for my MS-Pro on one side only and again I was expecting big sound differences between each side but could not identify clearly. I will redo the test on the SRH-940 with the Blue Tac and will also sew the other leather pad so I can properly compare these modifications. Cheer