Yeah, I’ve heard consistently that the 600 DAC is a big upgrade. Not sure if it’s worth the $4-5000 price difference between a used 590 & a new 600, though. E.g., I may be able to pick up a like-new PS Audio DirectWave (the first model, not the new MkII) for half that cost, including warranty.
It’s so tough having to buy these types of components with no opportunity to ever hear them first-hand. I miss the old days when there were hifi stores on every corner. But today, the closest dealers (and shows) would require a day in the air to reach.