OTOH, field coils are making a huge comeback right now. Active speakers are not likely to see innovations like that since they are closed systems.
Historically, the vast majority of field coil speakers were indeed used in "closed" systems . . . such as radios, instrument amplifiers, organs and Leslie cabinets, etc. etc. This had the added advantage of the amplifier being able to use the field coil as a power-supply choke - effectively working to reduce the cost of two parts (choke and speaker magnet).
The main push for the adoption of permanent-magnet speakers came after World War II, with the demand for larger separate speakers for movie theaters and music production - many the very earliest examples of these at first had separate field-coil power supplies. The introduction of Alnico V as a magnetic material (itself developed during WWII) was the main reason that field-coil speakers were abandoned.
But as always . . . audiophiles have very fickle preferances, and while some may find it interesting and comforting to experiment continually with the amp/loudspeaker relationship, it seems to me that a significant number of posts here on the Audiogon forums is by people who are asking advice on trying to get this right -- maybe some of them would enjoy a product where this was already done for them as part of the product engineering.