agree w @ghdprentice
from a practical p-o-v for users sorting through amp choices, the best indicator is if the maker clearly states the amp is able to double wattage from 8 to 4 to 2 ohms... another indicator, less good than the first, is if the amp (at least traditional class a or ab ones) are big and heavy with substantial power transformers and heat sinks - delivering consistently high current flow into low impedance loads generates mucho heat