I'd be curious to find out what problem got ironed out by a paleolithic design. I will confess to having heard that special SET magick a couple of times. It's all about the midrange.
My speakers (horns - active crossovers) are 107db/w efficient from 500Hz up, and this is the driver which uses the SET. I use the SET to eliminate any potential for crossover distortion - the audibility of this form of distortion has been a problem in the past. These drivers, coupled to the large horns, are so sensitive that
any low level grunge or noise upstream is audible. A PP class "A/B" tube amplifier or ultralinear amp would also be devoid of crossover distortion, but this topology would suppress even order harmonic distortion which then leaves the odd orders to irritate. The SET is heavily modified and uses feedback, so the harmonic distortion is typically <= .02% at most at the levels I listen at.
Also, since the SET is only capable of 2.5 watts, potential accidental damage to the diaphragms of the drivers is lessened.