Interesting, matches what I've been seeing and that is different than most other speakers.
https://forum.polkaudio.com/discussion/183402/polk-audio-sda-srs-placement
After reading the above I went and skimmed though the owner manual of the SRS/SDA, pictures show the speakers close to the wall and explain about some signal canceling going on but I did not get the 6-inches part, but can be assumed from the diagrams.
List:
https://polksda.com/manuals.shtml
I will say that I don't think that based on the years made that I've head any of the original SDA series speakers.
I probably have heard one of the monitors in the early 1980's and tower RTA's in early 1990's.
https://polksda.com/list.shtml
I've heard things before and after.
Now I'm very curious.
EDIT:
I'm reading this 1985 Stereo Review article now, very interesting.
https://polksda.com/srsreview.shtml