The goals of a system for music can be very different from person to person and a flat frequency response is not the only goal. It depends on the type of music, the amount of spl to be achieved and other factors that lead to enjoyment and not just measurements. Many tube amp people don't need to achieve reference level for their enjoyment and like the sound of even harmonic distortion. But, this is getting off track of this thread. Scientific principals is the foundation of audio but, creativity shapes the final product.