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.....:-) .

The more I consider the question, the more I like @oregonpapa's example of a cello or upright bass played with a bow. My sister played cello, and I've played with a couple of acoustic basses, so I'm pretty familiar with the sound they produce. The catgut strings on a bow, pulled across the strings of a cello or bass (or violin, for that matter; I've also performed with a couple of fiddle players) produces a very physical sound ( a "scraping"-type sound), one possessing lots of texture (think of running your hand across a deep pile carpet, or a loose-knit sweater ;-) .

Textural density relates to the amount of the "physical" sound of an instrument or voice, as in not just the sound a drum produces, but also the sound of the wood or plastic tip of a drumstick striking the drumhead. The friction between the stick and head creates a sound of it's own, a textural detail. Direct-to-disk LP's are superior in their ability to reveal such texture. Textural density is the opposite of "whispiness", which is the lack of physical "presence". A ghostly apparition, without physicality.

Good analogy @twoleftears. Art Dudley was fond of this kind of phrase as well, and both he and Herb used it in their reviews in Art's 90's mag, Listener. I've always taken textural density to mean a full-bodied sound, akin to color saturation in photography.