"But, I'm a little confused...Mapman says MMG are good on transients and attack, but Polarin says they are not great for rock because they lack weight and dynamics and Rpfef says they have greater impact (than the LS3).
How do I tie these opinions together to get an overview?"
Sum,
Attack is one aspect of transient response.
Transient response is just on aspect of dynamics having more to do with timing and speed.
Impact and weight are less technical terms but imply yet other aspects of the sound related to dynamics having more to do with magnitude rather than timing or speed.
Together, they help describe the sound and dynamics in different ways that are likely measurable.
Opinions are useful, especially from trusted sources, and in quantity but you never know the full story until you listen and compare.
How do I tie these opinions together to get an overview?"
Sum,
Attack is one aspect of transient response.
Transient response is just on aspect of dynamics having more to do with timing and speed.
Impact and weight are less technical terms but imply yet other aspects of the sound related to dynamics having more to do with magnitude rather than timing or speed.
Together, they help describe the sound and dynamics in different ways that are likely measurable.
Opinions are useful, especially from trusted sources, and in quantity but you never know the full story until you listen and compare.