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

"Warmth" is a very difficult characteristic to describe with adjectives. It's for me, the way real instruments sound in an acoustically neutral environment. Right now I'm listening to Strunz and Farah. They are both master acoustic guitarists. I have friends who play. The pluck of each string is there but so is the "woodiness" that the guitar body imparts onto the sound of each note as it is heard. A "lack of warmth" for me would be if the pluck of each string was unnaturally exagerated by a "hot" tweeter and the mid and lower mids were not able to deliver the woodiness of the instruments.

It can be a daunting task to get a system to this point, but is it ever worth the effort! Once you "tune" your system to achieve the inherent warmth of real instruments, the willing suspension of disbelief, can, with the right music, help create the illusion that you are indeed hearing the real thing. That would be the holy grail for me!

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. :)