I'm too interested in this question. I'm no engineer so this information may be incorrect however what I've been able to piece together is that higher effeciency speakers are generally superior at lower levels due to thermal compression in lower effeciency speakers.
More specifically because said TC doesn't affect all frequencies equally lower-eff speakers are tuned for a range of power dissapation. If your speaker was tuned to play louder and you listen quietly your performance suffers as you're not suffering an effect compensated for in design. By this thought it seems rational there will be exceptions though without recommendations on use from the manufacturer finding them may prove difficult. Higher-eff speakers have little to no tuning for this factor and thus are consistent accross a wider range of volumes.
More specifically because said TC doesn't affect all frequencies equally lower-eff speakers are tuned for a range of power dissapation. If your speaker was tuned to play louder and you listen quietly your performance suffers as you're not suffering an effect compensated for in design. By this thought it seems rational there will be exceptions though without recommendations on use from the manufacturer finding them may prove difficult. Higher-eff speakers have little to no tuning for this factor and thus are consistent accross a wider range of volumes.