Hi Bryon - for me, assembling a system that sounds "real" is automatically also going to be a system that sounds "warm." I suspect this is true for the great majority of audiophiles out there, especially those whose reference is live, unamplified acoustic music in a good performing venue (I am assuming, for instance, that this is what HIfiman means by "acoustically neutral," but I could be wrong). A good performing venue is "warm." I have never heard a system I would describe as "cold" or "harsh" that I could also describe as "real." Hifiman also speaks of the "inherent warmth of real instruments", which is certainly true, and I would also add the human voice to that. Though an actor may make his voice sound as cold as possible, there are extremely few instances where a singer would do so. Even HIP groups that don't use vibrato still have a natural warmth to their string tone. I really don't care how well a high-end system may measure, if it doesn't sound real/warm, and IMO/E, way too much high end equipment falls short. As for technical reasons why, I am certainly nowhere near as qualified as many others on this board to answer that; but this does put me in mind of something I read I think last night in a different thread where Atmasphere said, and I am probably badly paraphrasing here, that designers often have a choice between making equipment that measures well, and equipment that obeys the rules of human hearing. Perhaps he will weigh in on this thread.
Incidentally, part of the above is another instance of confusion that results from the use of the term "neutral." :)