First of all, what a coinkidink! Back in around 1970, I was being paid very little as an intern in a hospital, and I craved a pair of IMF Monitor loudspeakers. As a financial compromise, I bought a pair of IMF Studios, instead (from Lyric Hi-Fi in Manhattan). The Monitor used a KEF B139 in a Bailey-type TL, a KEF B110 midrange, and a KEF tweeter. Not entirely satisfied with the Studios, I eventually decided to try to build my own pair of Monitors from scratch. I had a patient who agreed to let me use his table saw, and I acquired two 4X8 sheets of HDF, about 1.2 inches thick, and lots of clamps, glue, screws, etc. I bought the B139s and B110s easily enough, and then I found a guy in California willing to sell me RTR ESL tweeters (the blue rectangular ones), as many as I wanted, with a power supply to drive them. I bought eight, with the intention of using four per speaker. Thus I built a TL speaker with the B139s and B110s in entirely separate TLs. (The woofer cabinet is nearly a carbon copy of the Bailey design, a la the IMF Monitor.) And the four RTR ESL tweeters were arranged in a linear array along one side of the woofer enclosure. A few months later, I met an EE who helped me design the crossover and in fact wound me some inductors. This was a darn good speaker that blew the pants off a pair of Magneplanar Tympani 1Us, when they came on the market. I moved from this speaker to full range ESLs shortly thereafter ( 2 pairs of KLH 9s), and I sold the home-made speakers to my cousin, who used them for at least 20 years. When he finally wanted to be rid of them, I bought them back, cut off the parts of the cabinets that supported the B110s and the tweeters, and stored the TL woofer cabinets in my basement for another 20 years. Then I bought the Bev 2SWs and finally saw a use for the B139s in the TL. (I had kept a pair of NOS B139s, all that time.) I still use one of those B110s as a test load when I work on my amplifiers. The B139 won't take much amplifier power, but I think that in a TL, it is one of the lowest distortion woofers ever made.
A tube amplifier with no output transformer is called an OTL. OTLs can drive any speaker, other than an ESL, with a high enough input impedance. What the Beveridge, the Modjeski, and other ESL "direct-drive" amplifiers do in addition is to drive an ESL with no step-up transformer at the ESL end of the chain. So, no transformer in the signal path, at all. To do this, direct-drive amplifiers have to develop thousands of DC volts at their outputs; the audio signal rides on the high DC voltage, which simultaneously biases the ESL. (The Bev speakers are a special case of an ESL where the diaphragm is low impedance; classic ESLs have a very high impedance on their diaphragms.) DD amps are OTL, but not all OTL amps can DD, is my point.
