What gets better as you go up in Audio Note DACs?

I've heard several people say that the Audio Note 3.1x balanced DAC is better than the 2.1x balanced or the 4.1x balanced is better than the 3.1x balanced, and the SuperDAC 5 is, well...!

The 2.1x balanced is really good. The best digital I've had in my system. So I'm wondering what gets better as you move up to the 3.1x and 4.1x? Anything that can be specifically defined? Or just an exciting sense of wow, this is better? I won't be able to demo these for a while.

Worth going from2.1x to 3.1x, or just go for the gold of the 4.1x? The 5 is out of the question.

Thanks folks.