Some of the Amaryllis will survive fine in zone 8 if they have some frost protection, like a blanket or piled up leaves. I suspect the thing that has the greater influence is the soil. If it stays too wet, that is a recipe for rot. Also microclimates make a difference. A spot near a wall that radiates heat at night might be enough to keep the bulb from freezing. It's a matter of trail and error. Considering the high initial cost of bulbs, most people would prefer not to leave matters to trial and error. That's why I (zone 9A) plant in big pots and throw blankets over the pots on the few nights that we go below freezing.
