I recall reading somewhere, roughly twenty-five years ago, that the B139 operated as a nearly perfect piston up to 800 Hz ballpark, at which point it began to have severe breakup modes. So the recommendation of the day was to cross it over no higher than 400 Hz. Unless your tweeter is a very widerange driver, you'll need a midrange unit.
My first pair of homebrew speakers used B-139s in a transmission line. I built them in 1979. Later I used B-139s in sealed, vented, aperiodic, and isobarik enclosures. I liked the driver a lot, but its x-max was fairly modest by today's standards as I recall.
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