It doesn't sound like a whole lot of progress has been made on containerless processing in the last twenty years. In 1990, as a student, I was working with a group at JPL doing pretty much the same thing. We had only a single driver ("speaker") instead of two, but were able to levitate a fairly reasonable weight with about ten watts of acoustic power if I remember correctly. Naturally, you need a lot less power if you don't need to overcome gravity.