IF you take a lower powered Amp, say 10 watts, running at .5% distortion and run it at 5 watts what is your distortion level compared to a high powered Amp, say 100 Watts and run it at 5 watts output, with .5%. That is not .5% at 5 watts but IS .5% at the capacity of the amp, or at a designated % of output. With a higher wattage capacity you can run the distortion right down to the floor. That is just one of the factors to consider.
This, or one could look at it from a different perspective: say you have an active speaker setup with no passive crossovers and potential steep phase angles - that's one way to effectively make the same amplifier "more powerful"/adding headroom for a given SPL or certainly making more effective use of its capacity with an easier load, all the while making it sound closer to its performing envelope - i.e.: better. Part of that is also dividing the workload between that amp and others to handle their respective and limited frequency spans; in a 3-way setup the amp handling MF/HF is freed of LF which equals lower distortion and better sound. The amp handling upper bass to lower/central mids is freed of everything else = better sound. Sub/central bass amp handles only that = better sound. You could have a blasting crescendo in the lower registers and it wouldn't matter an iota to the amps handling the other registers. With a line level crossover preceding the amps and looking directly into their driver segments with no interfering passive XO's also leaves out any influence of a dividing network seeing the full output power of the amp. All of this effectively makes a given amp more powerful and better sounding for a given context vs. the scenario of it seeing into a passive, not least complex crossover, full-range at that. Active requires more amps (unless running a full-range driver per channel), yes, but this is just to show how the same amp benefits from a given set of circumstances (incl. higher speaker sensitivity and impedance) when it's usually anything but impervious to load.