And the ES2 review - this one would be one of my choices for an IEM of the year short-list. Simply phenomenal.
Mate - how can it be. It's the same for both U6 and U10. How could it possible be any different. Its not a variable system - and measurements don't lie. What you and Linux4ever have shown though is how easy our brain can fool us if we believe something hsppens - when in reality it doesn't. It's called the placebo effect.
It'll drive the K612 loud enough. You won't be plugging your headphones into the fixed voltage line-out (you'll be plugging them into the headphone-out), so don't worry about it.
So USB to X3ii (if using Windows you'll need to load drivers). Headphones into X3ii. Simple.