What exactly is textural density??

It has to do with the music composition itself not the reproduction of it nor the playing,. This reviewer blabbering about textural density through headphones doesn't know what he's talking about.

“Realism” has everything to do with the correct expression of the naturally occurring harmonics (overtones) in the timbre of musical instruments. Audio equipment does not discriminate. If a piece of audio gear does not correctly reproduce the natural tonal (textural) density of a cello (to use bdp’s example) it will, likewise, not express the natural tonal density of an upright bass playing in the same range as a cello. They may each have different harmonic signatures overall, but any deviation from “realism” in a particular frequency range will impact another instrument playing in that range to one degree or another. Mahgister speaks to this Re “timbre”.

Djones, I agree with you, but I will give the reviewer the benefit of the doubt. “Tonal density” is a better term to describe what I think he is trying to say. Musicians use this term when discussing “timbre”.

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.