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

Some times, it could be as easy as swapping the source I/c. Not to alter freq response but take that electronic tinze/noise surrounding the notes. I have three sets of cables at each source, same corresponding brand for each level of sound i desire- neutral, slightly warm and warmer. You will be surprised how 'listenable' most music would become... true changing cables every once in a while is a chore and one set of cables is always your primary choice. It is no different than some folks trying different tonearms, cart, diif DAC sampling, diff sources...

One characteristic that warmer sound gets you is the image size ( overall- fundamental and harmonics and air) of instruments become life size which I personally consider primary requisite.

In 1970, a girl who I shared a house with, had a Fisher tube receiver and a pair of KLH bookshelf speakers that were given to her by a friend. Thats the first time I experienced "warmth" which totally had me captivated. Warmth is a full, rounded, refined relaxed sound that has a silky thickness and velvet texture to the music. A more of a full body to the sound where the upper frequencies seem to bend down more and blend and integrate into the mid-bands. From that era, several factor's come into play. Tubes at the time that were made by Sylvania, Zenith, RCA, and GE, were better built then the best tubes of today. Back then they had double glass envelopes, heavier filaments, and the technical artists who made those tubes are non-existent today. They were like composer's. In fact, the hardware machines are still in existence collecting dust at those companie's, but their is no one around today that has the technical ability to revive the technique and skill to make those tubes. A lost Art. The tubes from that time would last for ten years or more, as compared to tubes today that are good for 4K to 6K hours. Another fact is at the time, speakers had a lower sensitivity of 83 to 86 db's. Speaker's today with sensitivity between 90 and 96 db's, have a more aggressive open top end. Speakers with much lower sensitivity have a more relaxed character. Back then most amplification in homes was between 25 to 40 watts. Also, tweeters back then were more simple and basic, and it wasn't until the late seventie's when Phase Technology in Florida invented and built the worlds first ferrous oxide fluid filled tweeter which improved the dynamic range. For the most part, it was the tube designs from fifty years ago that gave us that warm, rich, full velvety sound. If those type of tubes
came back on the market, we would return to the full analog glory that has been lost.

its very simple. try attenuating the frequencies between 3 and 5 k by say 3db and increasing the spl by 3db between say 90 and 200 (i may be a little off here) and , voila! warmth.