Don't you think it's more difficult to optimize the positioning of large full range speakers in relation to the listening seat since the midrange, treble and bass drivers are all located within a single cabinet?
I don't consider it difficult to position large full range speakers to optimize the mid/treble frequency response and imaging at the listening seat; it typically just takes some experimenting with the distance between the speakers, distance out from the front wall, distance from the listening seat, toe-in and often some room treatments along the side walls to control first reflection points.
Once the large speakers are optimally positioned for midrange/treble and imaging, however, the bass response at the listening seat cannot be optimized because the position of the bass drivers cannot be independently located within the room to do so. The bass drivers are locked into position, usually at the bottom section of the large speaker cabinets below the midrange and treble drivers.
I agree with you that adding a sub or two to a system that uses small monitor speakers betters the experience by extending the low end that is otherwise missing.
But I disagree that a pair of good quality. large and expensive full range speakers would be superior to a pair of good quality, smaller and less expensive monitors on stands along with at least a couple of good subs. The main reason being the ability of the monitor system to independently optimize the mid/treble/imaging at the listening seat with monitor positioning and the bass response at the listening seat with sub positioning.
Of course, I think a pair of good quality, large and expensive full range speakers along with at least a couple of good subs independently positioned would be even better.
Tim