There’s flat frequency and then there’s the issue of spatial localization.
If your don’t care for spatial localization, a pair of Stax Headphones will give you perfect, flat FR.
While nice for headphone listening, it’s not what people associate with in-room ‘music’. So most listening spaces need some reverberant field cues for the brain to latch onto. Now, most rooms are waaay to reverberant and overlay their signature on the music to the point where listening to the performance is difficult.
A really good room will have some localization, but will have enough absorption and diffusion to not provide easily distinguishable acoustical distance detection from its surfaces. Basically, it should have some reflections, but not enough that our echo-location can map out the walls exactly. This is what makes a well treated room ‘disappear’ or sound ‘bigger’ than it really is. It fools the echo-location (which derives data from the delta between first arrival sounds and reflected sounds) into not ‘seeing’ the room for what it is.
Flat bass response is almost impossible to achieve in a small (3,500 cu ft) space. But one can get close, and when you do, it sounds cleaner (mostly due to lack of temporal ringing effects discussed above).
