I’ve spoken offline to several people who have owned both and the general gist of what they tell me is consistent: Although the H600 DAC is a big step up from the 590 DAC, there's not a huge difference between the 590's & 600's output in other ways. And the H390, while still a fine-sounding amp, is a bit outclassed by its two siblings in both analog & digital domains.
This is all just hearsay, of course, what other people are telling me. Maybe somebody here with first-hand experience can answer kennyC’s question more authoritatively than I.