If you look at measured frequency response of any audio equipment then you never see a truly flat response. At least I haven’t. And the phase shift of often far

If you look at measured frequency response of any audio equipment then you never see a truly flat response. At least I haven’t. And the phase shift of often far from flat. So if you have two systems that have near flat responses it’s possible that one sounds better for certain inputs. Also Bose plot (frequency and phase plot) is a type of analysis that assumes the system is linear. If the system is not linear, what is the plot showing you? I imagine many systems are very close to linear, but at the edges of dynamic range they can’t be.