Except for room aesthetics considerations, I don't understend why everyone doesn't orient speakers, especially ones that have smooth off-axis response, to have their axes cross in front of the central listening position, to widen the "sweet spot". This would seem to me to apply particularly to all the SP Tech speakers and others with properly implemented waveguides, since the roll-off to the sides occurs right down to the low end of the HF driver's operating range.
The idea is that as you move to the right from center, for example, and move closer to the right speaker, you are moving further off the right speaker's axis and closer to the left speaker's axis.
If the left speaker is slightly louder than the right speaker when the listener is slightly right of center, this can somewhat compensate for the arrival time from the right speaker being earlier than that from the left speaker and produce a more-or-less centered image.
This can result in a sweet spot 3 seats wide instead of one.
The central seating position is not compromised, either, if the off axis response is sufficiently accurate at the angle chosen.
In practice, many speakers sound and measure a little better at 5 - 10º off axis, anyway.
The degree of toe-in can be adjusted with a mono signal.