That aB in 104aB stands for acoustic Butterworth, which has to do with the mechanical crossover from the woofer to the passive radiator. Sticking an active B 139 in their doesn't make sense -- completely wrong box. While KEFs aren't perfect -- they achieve lack of coloration at the expense of efficiency and, hence, possibly lack of micro dynamics -- Ray Cooke certainly knew a lot more about what he was doing than most speaker manufacturers, and emphatically more than your average tinkerer. It's one thing to try to improve them by making them more efficient -- it may be wrongheaded, but at least it is an actual informed goal -- but to use an inefficient woofer in a too small box to replace a passive whose purpose was to raise efficiency while extending low bass is just counterproductive.
Certainly KEF was trying to make a less expensive speaker than a full 139 system (and the 104aB was also available as a kit), but they didn't go with a passive in the design out of cheapness or stupidity. Its a very nice speaker for its size and cost.
And that foam grill made perfect sense back in the 70s -- acoustically transparent, no diffraction causing frame, and cool looking (by the standards of the day). No one thought that people thirty years later were going to be still using the speakers, and so the fact that the grills decomposed is not really a blot on their honor.
