i have used a few words to describe this general concept of liquid or liquidity. maybe grain-less or continuous get closer to my intension as these two words don't infer some sort of added warmth or smoothing or rounding. gear can be grain-less, gear can be continuous, and not add warmth.....or very, very minor amounts of it.
Sound is difficult to describe. I thought you did well in your post. And if there is such a thing as liquidity, the Tenor OTL may be the best example of what can deliver it.

