Here's Linkwitz's quick overview of power response, which stated briefly is smooth, consistent output of all frequencies at different volumes.
http://linkwitzlab.com/speakers.htm
The best way to achieve this is usually with active crossovers and speakers (studio monitors) - like PMC, Genelec, ATC, etc. But often, people perceive these speakers to be "too" truthful/less musical, and as we all know, we often can't handle the truth. The 'problem' with studio monitors is that they reveal that often, [rock or other] music is recorded with dips and boosts in frequencies which are easily heard at loud volumes. (More cymbals and more cow bell :D). I suspect some Vandersteens have a declining power response as frequency or output rises.