As befitting my username, I'm still partial to the ESS AMT-1 pair I bought back in the 1970s.
Ah, yes, the air motion transformer - a great driver, even by today's standards. The first studio I worked at still used them. Best explanation to anyone who doesn't know how they work is like you squeezing a cherry pit between your fingers until it launches - that's what the AMT's are doing to air.