They are made by Classic Audio Loudspeakers. IMO/IME field coils have speed that otherwise you can only get from ESLs and for very similar reasons- both have power supplies to run their motive forces. In a permanent magnet system, when you put current through the voice coil, the magnetic field sags a bit. Alnico sags the least, and is easiest to focus in the voice coil gap, which is why Alnico is the preferred magnet system. Neodynium is the most powerful magnet system, but its harder to focus and sags the most.I am curious, what are your field coils, and how do you find them compared to other approaches.
Field coil OTOH doesn't sag- so there is less compression, higher speed and lower distortion, and I really don't think its hard to hear that difference. Permanent magnets fall into that same arena of most of the big changes in audio history; they didn't go away because they were inferior, they went away because they are more expensive. This was the same motivator behind tubes to transistors and certainly analog to digital.