The use of radial magnets is still quite unusual in loudspeakers because of the difficulty in magnetizing in situ and the consequent need for novel ways of introducing live magnets into the steel assemblies. However the benefits are manifold.
Vivid Audio leads the way in Tapered Tube loaded dome drivers and these rely on having the largest hole possible in the central pole to allow the sound to radiate freely into the absorber. So that excludes the use of a round disc magnet mounted within the voice coil. In addition to this there is always a benefit to be gained in terms of a smooth vertical dispersion by having the drivers close together. So that rather counts against having a ring magnet outside the coil.
A rear mounted radial magnet neatly answers both requirements while offering further benefits.
Because the poles of the magnet are polarized inside to outside rather that front and back, the design is self-shielding and can be used close to a traditional CRT monitor without further screening. Furthermore the naturally low leakage focussed field gives a higher maximum gap flux than a standard flat ring. A property which is exploited in the D26 tweeter which reaches a peak flux of 2.5T, roughly double that found on many 25mm tweeters, leading to a 96dB/W efficiency level.