The amp designers who post here would be best-qualified to answer your question. But it seems like full sound at low volumes is, at least in part, a design intention. For example, Bryston says of its 28B-SST2:
Is this merely marketing copy? Maybe. But they do sound both full and delicate at very low volumes, and maintain their tonal balance irrespective of volume levels. I'm not sure how much of that's attributable to their brute power, but the literature suggests not all of it.
A significant part of the design criteria for the 28B SST2 was to develop a very powerful amplifier that would drive any speaker on the planet but maintain an ideal power curve at 1 watt as well as at 1000 watts and every power level in between. Most amplifiers exhibit a power curve whereby the best noise floor, drive capability and distortion is maintained from about 1/3 power and up.
The Bryston 28B-SST2 Mono amplifier maintains its power curve right from the first watt. This results in a BIG POWERFUL amplifier that sounds incredibly detailed and musical at very low levels and maintains that same sophistication and drive capability with even the most difficult, inefficient speakers, large or small.
Is this merely marketing copy? Maybe. But they do sound both full and delicate at very low volumes, and maintain their tonal balance irrespective of volume levels. I'm not sure how much of that's attributable to their brute power, but the literature suggests not all of it.