Flat on-axis frequency response is clearly the engineering objective for most of these systems. Those that deviate significantly earn lower ratings in double-blind subjective evaluations. Although there is more to be considered, a flat direct sound delivered to listeners is the basis for most reproduced soun
The important takeaway here is 'flat direct sound', in other words flat when reverberating sound is not combining with the direct sound, as in an anechoic state or a free field, meaning no reflected signal or measured in room with gating employed during capture.
I personally strive for a smooth response, which is to say I avoid peaks and nulls by careful driver choice, crossover design and room treatment. Using 2 or more subs will also reduce the lumpy response.