I owned and sold Polk's original SDA speakers in the mid 80s. While it appears there have been improvements, the overall concept seems the same.
Back then, we only detailed the technology to those who were very interested. The benefits of SDA are immediately apparent when the speakers and listeners are positioned correctly. The width of the stereo image expands well beyond the distance between the speakers, and even the room walls. If I remember right, we were provided an A/B switch that eliminated the effect, to demonstrate the difference. I do not know whether it simply disconnected the A1 SDA interconnect cable, or switched its polarity.
POLKSDA.com is a site dedicated to the original series. It links to
this in depth 1985 article in Stereo Review describing the technology. I owned the compact
SDA CRS model , proving that SDA can be put into a stand mount speaker.
While impressive, I did think the effect seemed somewhat artificial...not tied too closely to the intentions of the recording engineers.