What is “warmth” and how do you get it?

I think most can agree that reproduced music has warmth when violins dont sound like slaughtering poultry and piano transients dont sound like someone hitting your temple with a punching tool:

So how do I pick a pc of gear of mod a pc of gear to make it more warm?

To quote some ideas from Scott Frankland:
Warmth usually comes with
* regulated power supplies
* non inductive resistors (carbon composite)
* low odd ordered harmonics

and I would like to add to that:
* oil filled caps, be it coupling or bypass
usually they come with a more linear top end response to dynamics and a richer midrange (2nd order harmonics?)
* correct coupling cap orientation
* triode power tube config (to me Ultralinear never sound warm cos its actually anything but linear at least to my ears)
* use of chokes in power supply filter (that minimizes the unwanted spiky transient distortions from complex passages)

* Ribbon tweeters are normally NOT warm.

Warmth also comes with coherence. A poorly designed crossover will not give you the bloom and palpability of instruments that reaches out to you without attacking you when playing a wide range of pitches.

My 2 cents worth.

I have no idea how you 'make' warmth, but to me it sounds full bodied, textural, with a slightly plump bottom end, strings have bite, the top end is airy and delicate, without any hint of harshness. Too many components today are the opposite - strident, lean, clinical.

I guess aside from good guts, it's in the tuning by the designer.

Just a personal experience. I was at a high end salon sitting in a chair while they were setting up a system. They substituted various components and I could tell the difference as they did. The $2k cd player (Arcam) sounded fine but replacing it with a $3500(Linn) added a 3rd dimension to the soundstage. The speakers were the ones with the Be tweeter. The preamp was a tubed Audio Note 1 ($1k). Then they substituted a Linn ($1500) solid state preamp. The sound became shrill and I was pretty much driven out of the store. It wasn't even close. So warmth to me was tubes that made me want to listen indefinitely. I think they did that on purpose when they figured out I wasn't buying that evening. :)

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." :)

Play "Light My Fire" over and over again.