I cannot remember where I read it, but the suggestion was to do one’s tweaking of individual instruments/vocals on a single speaker, and not mono in two speakers.
The single speaker makes it easier to concentrate on just the quality of the sound rather than being distracted by effects from the space between two speakers. It makes the quality of the sound less dependent upon the level and room effects.
Phil Spector was an expert in rich tapestries of sound. However, his material is rather dense for inexperienced people to de-construct enough to get an exploded view of the tracks. I think the 60s had many examples of simpler constructions that are much easier to decode.
However, if one is trying to hone one’s tweaking skills, it may be better to reduce the number of variables and mix for a spatial arrangement that matches a conventional band layout, with which one can compare similar well known mixes of the genre.
A good book about how many of the effects in some early hits were done without high-tech is Mark Cunningham’s good vibrations.