Smoothness. Smooth, not dull; clear, but not etched. Smooth. Like real acoustic music.
When I changed to film and foil caps, I heard smoothness. When I changed to nude Vishay resistors, I heard smoothness. When I changed to an air bearing TT, I heard so much smoothness that I suspected that something had been lost - and it was - I lost the need to stop listening after 2 or 3 hours. Improved motor isolation - smoother. Panzerholz wand - smoother. Koetsu - smoother.
How do I know that it's really an improvement? I listen to a song in another language, or in dialect, and I can understand it more easily. I disentangle the lines in a symphony more easily. Clarity without compromise. Smoothness.