Johnk is not wrong. High efficiency speakers are much more dynamic sounding, at least in part, because they suffer less from thermal compression. When playing at any given level, the higher efficiency speaker will be passing much less current and the voice coil will not be heating up as much as is the case with a low efficiency speaker. That heating of the wire causes an increase in the resistance of the wire, which means, when there is an even stronger current that is suppose to flow, the amount that can flow is reduced and not proportionate to what is supposed to flow. This results in compression (less than proportionate increase in the current flowing and the resulting movement of the diaphragm).
The increase in dynamics of high efficiency horn systems is evident even at quite low volume levels. In fact, I tend to find that it is MOST evident when playing music at modest levels--high efficiency systems sound more lively.