The Mirage M-1 used the sleeve and top cap in 1987. The speaker was wider and not as deep, five ft tall and weighed about 130 pounds. Definitive's website says their first speaker, a bipolar, came out in 1990. Paradigm's bipolars also used the sleeve-top cap. I think it was just a matter of design sense. When you have multiple speakers firing forward and backward, a sleeve would be the most logical way to go.