Achieving the disappearing act is a combination of all 3 things: speaker design, your room, and placement. If you have a speaker with an identifiable coloration, like a hot tweeter or a cabinet resonance, then the speakers will never disappear. Your brain will identify that coloration as part of the speaker and not the music. Same thing with early reflections. If you have a reflective surface on one side, and books on the other side, your room will be unbalanced, and good imaging will be difficult.
Speakers "Disappearing"
I have read a lot about speakers "disappearing" so that one can't tell from where the sound is emanating. But, what about all the stereo tunes where the recordiing engineer intentionally pans the music to come from one side or the other? Can the speakers be made to "disappear" in that situation? Or, is it just the nature of the particular recording?