When looking for an answer you need not only to open your eyes, but your mind as well.
From chapter 7:
In monophonic tests, (10, including Toole and possibly Olive) listeners reported large differences in both sound quality and spatial quality.
However, in stereo listening most of the differences disappeared in these data that average ratings for all programs.
The two highly rated loudspeakers kept their high sound quality ratings, but the loudspeaker with low spatial ratings in mono became competitive in stereo.
This was a puzzle, because it had been assumed that it was stereo that would reveal the relative merits in terms of imaging and space.
If the Quad was judged subjectively worse in mono both in terms of spatial as well as sound quality but almost identical in stereo then we can conclude that since no one listens at home with a single speaker then perhaps the problem is not the narrow dispersion but something else.
Perhaps people when listening in mono preferred the less flat reponse of the other two speakers because it introduced an euphonic quality since both the Rega and the Kef exhibit the infamous BBC-dip at 2kHz.
Or maybe I am just a herectic infidel that should be set-alight.