Nice you have discovered the Field Coil Full Range drivers, they are the best speakers there is. Field Coils were the first commercially available speakers drivers to the public in the early 1930s.
Years later appeared FR speakers with Alnico 5 magnets which are the second best and also are nice sound, the third best speakers are Alnico 8 magnets, but after 1945 WW2 all Cobalt used in Alnico magnets become awful expensive and rare due having been reserved for military use in nuke bombs.
Current speakers with ferrite magnets be Woofers or FR used in 99% of speakers roday are truly mediocre sound and are in a distant fourth position, their raw dry sound, poor in harmonics is depressing, so continue with FC drivers or Alnico 5.
If you have a hi sensitivity speaker as 100dB and bening impedance curve there is no much things to upgrade IMO, maybe inside the enclosure if you are picky as myself. Today audiophiles can take advantage of inexpensive and transparent powerful Class D or GaN-FET amps and recently with Lii Song Audio, also take vantage of inexpensive big cones full range drivers, this will save a lot of money hence no crossover and no enclosure need.
This is a inexpensive DIY project from an AC member,
It have only four (4) parts:
Eminence Alpha15A + 4.7mH + Visaton B200 and a MDF baffle.
Other option are the Lii Audio F15 FR driver $399/pair, not each.
Here Decware OB with the Lii Audio F15 driver:
If one have a woodworker is possible try the Lii Audio F15($399/Pair, pair not each) 97dB/8Ω and benign impedance curve suited to OB or Bass Reflex box or the F18($540/pair), no Crossover, no Tweeter beaming, no Phase rotation, no driver sensitivity loss, no Harmonics loss, no Retail price.
http://www.glowinthedarkaudio.com/lii-audio-f18.html
My personal choice is the Lii F18 in Single Driver configuration, but if you make question of own a 2-Way speaker you may want a bigger woofer than the Alpha15A, so there are W18 inches and W21 inches from Lii Song Audio, personally I dont like these two VCs woofers, they are hard to drive with a small tube amp, they have two Voice Coils.
https://www.lii-audio.com/product/w-18/
https://www.lii-audio.com/product/w21/
Please note some Circles of interest you may like>
The Fullrange Driver Circle:
https://www.audiocircle.com/index.php?board=128.0
The Open Baffle Circle:
https://www.audiocircle.com/index.php?board=90.0
The Tube Amps Circle>
https://www.audiocircle.com/index.php?board=144.0
