What exactly is textural density??

Another term used in place of textural density---in particular by Sam Tellig---is "more 'there' there". The greater the textural density, the greater will the timbre of an instrument or voice be revealed.

Ray Charles', Howlin' Wolf's, and Aretha Franklin's voices have much more of it than do that of, for example, Brian Wilson, Graham Nash, and Karen Carpenter. We white's may benefit from some advantages, but when it comes to singing.....:-) .

What? No mention of Bing Crosby's tonal density? He has it in spades. :-)

I think we are conflating a couple of things. I don’t think this is about which voice or instrument has, in real life, more or less textural (tonal) density than another. While different voices or instrumental sounds obviously have different harmonic signatures, some richer in harmonics than others, as concerns the use of the term in audio (“more there there”), the point of quantifying or qualifying tonal density is to determine whether the reproduced sound does justice to the timbre of the sound being reproduced; whatever that might be. It is possible to have a sound that has “more there there”, as the phrase is used in audio, that is a sound that is TOO tonally saturated. Error of harmonic commission as opposed to error of omission. In my experience the opposite is the case more often than not....not enough tonal density.

Also, let’s not confuse tonal purity with lack of tonal density. I would argue that Karen Carpenter (or Bing Crosby) has no less harmonic density in her voice than Ray Charles. More nuanced with better balanced and integrated harmonics, yes. Less? I don’t think so.

I don’t think this is about which voice or instrument has, in real life, more or less textural (tonal) density than another......
the point of quantifying or qualifying tonal density is to determine whether the reproduced sound does justice to the timbre of the sound being reproduced; whatever that might be.

Right to the point!....

Tonal and timbre density and his texture to the perceiving ears is the most important perceived fact to evaluate our own system and any system.,...There are others but without this one any system is more trash than audio.... The unbeknownst fact to most is that implementing  controls of the working dimensions of the audio system is on par with a good electronic design choice to begin with.....

Herb is describing a richness of sound and detail accentuated in some pieces of music and lost or retained by certain combinations of gear/cabling/AC purity, I think. Interesting to note the musicology of his terminology, but no need to fault him for his resourceful prose, even if not textbook.  I always enjoy his reviews as much for his prose as his clear lack of corporate sponsorship.  We also seem to have similar tastes, so it makes finding good value gear easier.