I'd label it neutral, and damn fast. Outside of that I freely admit that I'm not one to hunt endlessly for the last 1% of purity. It's among the best digital I've heard for sure which puts it up there with the likes of Wadia, Electrocompanient, The Hertsens and others. Could you find something that you might like better, I'm sure of it, but I'd guess it'd be a pretty subtle preference.
And no worries about synergy with tube amps and HD650s (although I'm going to dismiss the "tube" aspect), I'll I've really used it with is The Menace and my HD650s. By in large I don't find the Menace to sound like a tube amp so hence the dismissal, it just sounds like a good amp.