I’m not going to gush over the improvements offered by the second amp as in overall terms they are not massive.
But isn't it the incremental improvements that sometimes make the biggest difference? Take speaker cables for instance - a person who doesn't do this for a hobby might not notice much of a difference between a $400 cable and one three times its price, or even 10x the price. But for you and I, that might make a huge difference in the way we experience the music. That concentration and depth of image that improves the overall presence of the music makes the difference worth spending, to us.
Anyway, I'm encouraged by your experience with two of the amps. (And have always wanted to listen to ATCs but have not yet done so.)