This is a question that I've posed to Outlaw as well.

In general, the main considerations when making this decision are room size, speaker efficiency, and the volume at which you play movies and music.

In modestly sized listening rooms with moderately efficient speakers, the 7100 will play plenty loud. The 755 and 770, however, will render dynamic contrasts in the material better at higher volumes.

So, for inefficient speakers, larger listening areas, or higher volumes, the larger amps are recommended. Otherwise, Outlaw considers the 7100 to be adequate.